Install nova (master/git) in testing environment

As we're a plugin, we require several imports from nova.
Thus, it is important that we have those modules available in
our testing environment.

Unfortunately, it seems that bringing this in via the requirements
files is unreliable so we instead include this directly via the tox.ini
where we can specify the '-e' parameter.

Change-Id: Ia5aa9d91fe5c263f5c8d492f9335c6e9adb191eb
Merge "Install nova (master/git) in testing environment"