Fix the layout for "Confirm Subscriptions" screen.

Firstboot on large screens was making this dialog
expand across all the available vertical space.
Fix layout issues with select sla screen in firstboot.
Allow test_managercli.py to run without a display.

The test_managercli.py class was importing from
test_handle_gui_exception.py which ultimately imported gtk.  Gtk
will fail to import if there is no display available so we were
unable to run test_managercli.py while SSHed in.  This commit
moves the shared classes to fixture.py
Add 'override' command to alter content repositories server-side.
Created Repository Management Dialog
No longer make multiple calls to overrides API
Properly convert an int/bool string override value in UI
Rename "refresh_overrides" to "cache_only"

Also remove unnecessary cache class.
Handle incorrect arguments to override command correctly.
Correcting stylish error.
Created an overrides module

Provides a library for accessing modifying Content Override
data from the server.

CLI and GUI now use this module.
Made Repository Details resemble Subscription Details

Change was made to make things a little more consistant
with the current GUI. Also made a few alignment fixes
on the repository details panel.
Fixed code style issues
Fixed search and replace typo.
Added repo dialog tests
Show message instead of repo table when no repos exist.
Fixing tests to comply with GTK version
Updating tests to be compatible with older python
Updated Repos dialog no repos text
Updated Repos dialog no repos text format