Added use of trusts to Physical Host plugin

Now the physical host plugin uses trusts to communicate with Nova.

Added a decorator in order to create a trust in certain API calls.
Added the trust_id to the ComputeHost model. Added a DB migration for
this change and a unit test for it.
Added the MissingTrustId exception, which is raised when no trust_id is
provided to some methods of the RPC API.
Removed all the configuration keys related to the admin Climate user.
Modified NovaClientWrapper in order to use the information stored in
context.

Change-Id: I0b83a933c0d72871654f3c6252be5d5e2c4cfd54
Closes-Bug: #1285585