Remove V3 API version of coverage extension

As discussed at the summit in the QA session, this removes
the coverage extension from the V3 API. The coverage extension
doesn't work properly and has security issues. An alternative technique,
though more limited in nature will soon be proposed.

Partially implements blueprint nova-v3-api

Change-Id: Ib4296288fefeb7e1e11bb6e44bd1cf355f29b9b9
Merge "Remove V3 API version of coverage extension"