Remove duplicate FlavorNotFound exception handling in server create API

The v2 and v3 openstack server create APIs have duplicate handling for
the FlavorNotFound exception. This patch removes the duplicate handling
which is found in a generic catch-all block for each API.

Closes-Bug: #1256427

Change-Id: I78344cd588178c501398147f3afcd418e0744b0d
Merge "Remove duplicate FlavorNotFound exception handling in server create API"