Fixed nova-manage argument parsing error

In the dictionary CATEGORIES of file nova-manage, both 'flavor'
and 'instance_type' refer to InstanceTypeCommands, so the category
would be handled twice by add_command_parsers(). It would mess the
arguments if a command category is handled more than once by
function add_command_parsers().
Added a check to avoid the mess.

Change-Id: I7a314a9b389b3dea848a7111cedfc762bbb6b30e
Fixes: bug #1098733
Merge "Fixed nova-manage argument parsing error"