List Nova and Cinder extensions in debug log

For debugging purposes and post-processing Tempest logs it is useful
to get a list of Nova and Cinder extensions. This patch logs every
found extension to the debug log as a comma-separated list.

Change-Id: Ib550913e6c7b8b9eef12569c0ad7022c44b1f84d
Merge "List Nova and Cinder extensions in debug log"