[UG] Client configuration file

Adds information about Fuel client configuration file to
the CLI section of the User Guide.

Change-Id: I14a38650bc2a1fac808f3da5da7e9b282ca226b5
Closes-Bug: #1557443

userdocs/fuel-user-guide/cli.rst

@@ -3,7 +3,8 @@
 Use the Fuel CLI
 ================
 
-Using the Fuel Command Line Interface (CLI), you can:
+The Fuel CLI is a command-line interface for Fuel that installs automatically
+when you install Fuel. Using the Fuel CLI you can:
 
 * Operate your OpenStack environments using Fuel text commands, as well as
   using standard Linux commands.
@@ -25,6 +26,7 @@ This section includes the following topics:
 
    cli/cli_acronyms.rst
    cli/cli_basic_usage.rst
+   cli/cli_client_config_file.rst
    cli/cli_management.rst
    cli/cli_environment.rst
    cli/cli_deploy.rst

userdocs/fuel-user-guide/cli/cli_client_config_file.rst

@@ -0,0 +1,52 @@
+.. _cli-client-config-file:
+
+Modify the Fuel CLI configuration file
+--------------------------------------
+
+The Fuel CLI uses the ``fuel_client.yaml`` file as a source for default
+settings. By default, Fuel stores the ``fuel_client.yaml`` file in the
+``~/.config/`` directory.
+
+**To change the default directory:**
+
+#. Log in to the Fuel CLI.
+#. Set the required directory path:
+
+   ::
+
+     $ export XDG_CONFIG_HOME=/path/to/fuel_client.yaml/
+
+   where ``XDG_CONFIG_HOME`` points to the ``fuel_client.yaml`` file directory.
+
+**To specify custom settings:**
+
+#. Log in to the Fuel CLI.
+#. Edit the ``fuel_client.yaml`` file.
+
+   Alternatively, create a new YAML-formatted file:
+
+   #. Create a ``.yaml`` file with the required settings.
+   #. Export the ``FUELCLIENT_CUSTOM_SETTINGS`` variable:
+
+      ::
+
+        $ export FUELCLIENT_CUSTOM_SETTINGS="~/custom.conf"
+
+      where ``"~/custom.conf"`` is the path to the new configuration file.
+
+   #. Optionally, add the export to the ``.bashrc`` file:
+
+      ::
+
+        $ echo 'export FUELCLIENT_CUSTOM_SETTINGS="~/custom.conf"' >> ~/.bashrc
+
+.. note::
+
+   Custom settings override the default ones. Top-level values may also be set
+   as environment variables.
+
+   **Example:**
+
+   ::
+
+     $ export SERVER_PORT=8080