Update docs for Ubuntu 20.04

* Add gettext package for development environment
* Use 'apt install' command instead of legacy 'apt-get'

Change-Id: I2bd06b7708a72a1986e75968aee79ba27457bf62

doc/source/contributor/quickstart.rst

@@ -19,13 +19,13 @@ On Ubuntu
 
 .. code-block:: console
 
-    $ sudo apt-get install git python3-dev python3-pip
+    $ sudo apt install git python3-dev python3-pip gettext
 
 On RPM-based distributions (e.g., Fedora/RHEL/CentOS)
 
 .. code-block:: console
 
-    $ sudo yum install gcc git-core python3-devel python3-virtualenv
+    $ sudo yum install gcc git-core python3-devel python3-virtualenv gettext
 
 .. note::
 
@@ -67,12 +67,6 @@ can be customised by modifying the ``OPENSTACK_HOST`` and
 Horizon uses ``tox`` to manage virtual environments for testing and other
 development tasks. You can install it with
 
-.. code-block:: console
-
-    $ pip install tox
-
-or
-
 .. code-block:: console
 
     $ pip3 install tox

doc/source/contributor/testing.rst

@@ -39,7 +39,7 @@ Selenium will use a virtual display in this case, instead of your own. In order
 to run the tests this way you have to install the dependency `xvfb`, like
 this::
 
-    $ sudo apt-get install xvfb
+    $ sudo apt install xvfb
 
 for a Debian OS flavour, or for Fedora/Red Hat flavours::
 

doc/source/install/from-source.rst

@@ -66,7 +66,7 @@ GNU ``gettext`` tool is required to compile message catalogs.
 
 .. code-block:: console
 
-  $ sudo apt-get install gettext
+  $ sudo apt install gettext
   $ ./manage.py compilemessages
 
 Static Assets
@@ -172,7 +172,7 @@ Deployment
 
    .. code-block:: console
 
-     $ sudo apt-get install apache2 libapache2-mod-wsgi
+     $ sudo apt install apache2 libapache2-mod-wsgi
 
    You can either use the provided ``openstack_dashboard/wsgi.py`` or
    generate a ``openstack_dashboard/horizon_wsgi.py`` file with the following