Use constants for microversion values

We very often end up with merge conflicts for any patches that increment
microversions due to conflicting numbers. We can't really solve that,
but we can avoid the need to update version numbers throughout the code
by defining a constant value in one place and using that variable instead.

This is originally done in Cinder and could be useful to adopt it in Nova:
https://review.openstack.org/#/c/505420/