Port certificates API to v3 Part 2

This patch contains the changes required to adapt the certificates
extension and the corresponding unittests to the v3 framework

Partially implements blueprint nova-v3-api

Change-Id: If4fe7e8524ba0181b50722281f594e0a80cd49ed
Cleanup certificate API extension

Fixes a couple of minor issues with the v2 and v3 versions
of the certificate API extension identified during the
v2->v3 port
* Removes unused CertificatesTemplate class
* Corrects doc strings for a couple of methods

Change-Id: If022d2393ceab5ce7e6311c92a6f52b6325039bc
Merge "Port certificates API to v3 Part 2"
Merge "Cleanup certificate API extension"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp