Fix l3 agent scheduling logic to avoid unwanted failures

In case router is being added to l3 agent which is already hosting
the router it is fine to let such a request to succeed.
This patch also adds a check for unnecessary scheduling that might happen
twice in described case and lead to unwanted messages in the logs.

Change-Id: Id104b36ba7e1e6f6a9378ee600c33e9962230521
Closes-Bug: #1358636
Merge "Fix l3 agent scheduling logic to avoid unwanted failures"