Rename Host to Allowed Host

To avoid confusion between nova hosts and
database user host e.g. as used in a grant
statement we will rename that one to
'Allowed Host'

Updated launch dialog and users table to
use 'Allowed Host' consistently.
Updated launch dialog help text to clean
up some of the descriptions

Change-Id: Ice39cd6712eb757a4d0f5beaedcda580fbeb4acb
Closes-Bug: #1342134
Merge "Rename Host to Allowed Host"