Change plan_uuid to plan_uri in assembly

This is what we have at the moment, and we should support plan's being elsewhere.

-Modified test asserts as they were passing even with the change.
-Update the help command output with the 'Assembly Create' change

Co-Authored-By: Murali Allada <murali.allada@rackspace.com>
Change-Id: I7a1dccadb55138ad461cc830f4c35f5c5920857b
Updating the help message

Updating the help message to reflect the changes in CLI commands
and the addition of the new 'Assembly get' command.

Change-Id: I02b5df406c66837a0c548f70fa4c1912aa70f2fe