Handle API limit exception in OS::Trove::Instance

troveclient raises a RequestEntityTooLarge exception when an API limit
is encountered.  Previously, this would cause the stack action to fail.
Log a warning instead.

blueprint improve-api-polling

Change-Id: Ia7e5e321d5e1284880ee71fa0457be5726860aad
Merge "Handle API limit exception in OS::Trove::Instance"