Add install guide for users and clean up README

* Add "Using Horizon" section to the docs, which describes
  how to install and configure Horizon. It is for operators
  who install Horizon from source code.
* Clean up README.rst as much of the information is out-of-date
  and most of it is covered by the developer docs.
* Minor update of page titles.

Change-Id: I09aab0d0bc47e07c8d663e7f7b5801e456434950
Fix for "No existing metadata" in Horizon metadata selector.

In path 'Admin' -> 'Image' -> 'Update Metadata'

If all metadata items are added to "Existing metadata",
"No existing metadata" will be shown in "Available Metadata" menu.

This patch replace "No existing metadata" string to "No available
metadata", to make situation clear.

Change-Id: Ie04ca6c444e149860ccb6fc059fad3d770c6631f
Closes-Bug: 1369248
fixed errors when run_tests.sh --makemessages

The change Ibf2036d5bbc4c478fcb912f6335104356e8f1317 causes
run_tests.sh --makemessages failure. Make the errors go way.

Closes-Bug: #1379054
Change-Id: I66b68099561bdd451321c7c31391f30827121e7c
Merge "Fix for "No existing metadata" in Horizon metadata selector."
Merge "fixed errors when run_tests.sh --makemessages"
Merge "Add install guide for users and clean up README"
Merge branch 'master' of https://github.com/openstack/horizon