nova-net: add more useful logging before raising FixedIpLimitExceeded

The OverQuota exception that we get back from the DB API contains some
useful information about the quota limits for the project and resource
and the current usage, so we can log that before raising
FixedIpLimitExceeded. Also logs the actual project ID passed to
quotas.reserve rather than context.project_id since it's possible those
are different.

This should help us figure out if we're leaking resources in gate runs.

Related-Bug: #1353962

Change-Id: Ia8a6d2336ddbbe28178be8d3ce4d97e6d4ab3787
Avoid using except Exception in unit test

Failed assertions raise AssertionError, which is a subclass of Exception.
Catching Exception and ignoring it then ends up ignoring any failed
exceptions caught by the exception handler.

Using a more specific sentinel exception prevents the ignoring of failed
assertions from happening

Change-Id: I93b3725e16adc16457580d5150f3d88a86d614dd
Closes-Bug: 1378088
Merge "nova-net: add more useful logging before raising FixedIpLimitExceeded"
Merge "Avoid using except Exception in unit test"