Make metadata server use objects for Instance and Security Groups

This makes the metadata service fetch an Instance object to
do its work, and use the SecurityGroupList object to avoid
calling directly to conductor's API for those two things.

Related to blueprint compute-manager-objects

Change-Id: Ia12f48227eb2380f5da93313cd4045577d8857c9
Merge "Make metadata server use objects for Instance and Security Groups"