Fix get_flavor_id() when flavor_ram is specified

Without this fix, _get_flavor_id() fails to find a matching flavor if
both:
* the flavor_ram parameter is specified
* the first flavor in the list does not match.

The bug is simply that the module.fail_json() call lies within the loop
iterating through the flavors.  This call should only be made if the
loop completes and no matching flavors have been found.