Avoid hard dependency on python-coverage.

Updates the coverage extension in the OpenStack Compute API to
avoid a hard dependency on the python coverage module. We now
try to import 'coverage' and if it isn't present we return
an HTTP 503 (service unavailable error).

This gets rid of WARNING errors which occur when running Nova API
when the 'coverage' module is not available:

2013-01-27 17:10:28.938 16018 WARNING nova.api.openstack.compute.contrib
[-] Failed to load extension
nova.api.openstack.compute.contrib.coverage_ext.Coverage_ext: No module
named coverage

Fixes LP Bug #1101057.

Change-Id: I573180aead44d68b8377cd66f30fc6ac849d0272
Merge "Avoid hard dependency on python-coverage."