Add replication slave info to instance show

Update the 'show' response for an instance:

 - If the instance is a replication slave, include the slave_of id.
 - If the instance is a replication master, include the list of slave
   instance ids.

Update the 'list' response to include the slave_of id.
Add CheckInstance tests for slave/slave_of info.

(Unrelated: remove a #TODO comment that has already been addressed.)

Partially Implements: blueprint replication-v1
Closes-Bug: #1340359
Change-Id: If8a154083d0095606fb3ee115cc9b66ab788cbb4
Merge "Add replication slave info to instance show"