novaclient 2.27.0 introduced the API microversion discovery and client.Client
now wants an api_version argument to properly work out the correct API
version in use. OSC needs to provide this when required.
Letting the compute client plugin do the version validity checking makes more
sense than encoding it into shell.py, so I've added a new OSC plugin interface
function check_api_version() that is called from shell.py if it exists. If it
either does not exist or it returns False the previous version checking using
API_VERSIONS is still performed.
compute.client.check_api_version() conditionally imports the new
novaclient.api_versions module and uses it if successful. Otherwise
check_api_version() returns False and the previous code path is resumed.
One side-effect of this is that it is now valid to use --os-compute-api-version
with any valid microversion supported by the installed python-novaclient.
Closes-Bug: #1492467
Change-Id: I4535b38a5639a03a9597bf83f6394f9bb45c2b9e
3abfea083a