Adds FAQ content to the Murano docs

This patch adds FAQ to the Developing Applications
section.

Co-Authored-By: Ekaterina Chernova <efedorova@mirantis.com>
Co-Authored-By: Olena Logvinova <ologvinova@mirantis.com>

Change-Id: I019cbe6ed14fc4e8db02b7643cdf57e320d75167

doc/source/draft/appdev-guide/faq.rst

@@ -3,6 +3,80 @@
 .. toctree::
    :maxdepth: 2
 
-====
-FAQs
-====
+===
+FAQ
+===
+
+**There are too many files in Murano package, why not to use a single
+Heat Template?**
+
+  To install a simple Apache service to a new VM, Heat Template is
+  definitely simpler. But the Apache service is useless without its
+  applications running under it. Thus, a new Heat Template is necessary
+  for every application that you want to run with Apache. In Murano,
+  you can compose a result software to install it on a VM on-the-fly:
+  it is possible to select an application that can run under Apache
+  dynamically. Or you can set a VM where Apache is installed as a
+  parameter. This way, the files in the application package allow
+  to compose compound applications with multiple configuration options.
+  For any single combination you need a separate Heat Template.
+
+**The Application section is defined in the UI form. Can I remove it?**
+
+  No. The ``Application`` section is a template for Murano object model
+  which is the instruction that helps you to understand the
+  environment structure that you deploy. While filling the forms that
+  are auto-generated from the UI.yaml file, object model is
+  updated with the values entered by the user. Eventually, the Murano
+  engine receives the resulted object model (.json file) after the
+  environment is sent to the deploy.
+
+**The Templates section is defined in the UI form. What's the purpose?**
+
+  Sometimes, the user needs to create several instances with the same
+  configuration. A template defined by a variable in the
+  ``Templates`` section is multiplied by the value of the number of
+  instances that are set by the user. A YAQL ``repeat`` function is
+  used for this operation.
+
+**Some properties have Usage, others do not. What does this affect?**
+
+  ``Usage`` indicates how a particular property is used. The default
+  value is ``In``, so sometimes it is omitted. The ``Out`` property
+  indicates that it is not set from outside, but is calculated in
+  the class methods and is available for the ``read`` operation from
+  other classes. If you don’t want to initialize in the class
+  constructor, and the property has no default value, you specify
+  ``Out`` in the ``Usage``.
+
+**Can I use multiple inheritance in my classes?**
+
+  Yes. You can specify a list of parent classes instead of a single
+  string in the regular YAML notation. The list with one element is
+  also acceptable.
+
+**There are FullName and Name properties in the manifest file. What’s
+the difference between them?**
+
+  ``Name`` is displayed in the web UI catalog, and ``FullName`` is a
+  system name used by the engine to get the class definition and
+  resolve the class interconnections.
+
+**How does Murano know which class is the main one?**
+
+  There is no ``main`` class term in the MuranoPL. Everything depends
+  on a particular object model and an instance class representing the
+  instance. Usually, an entry-point class has exactly the same name
+  as the package FullName, and it uses other classes.
+
+**What is the difference between $variable and $.variable in the class
+definitions?**
+
+  By default, ``$`` represents a current object (similar to ``self``
+  in Python or ``this`` in C++/Java/C#), so ``$.variable`` accesses
+  the object field/property. In contrast, ``$variable`` (without a dot)
+  means a local method variable. Note that ``$`` can change its value
+  during execution of some YAQL functions like select, where it means
+  a current value. A more safe form is to use a reserved variable
+  ``$this`` instead of ``$``. ``$this.variable`` always refers to an
+  object-level value in any context.