Starting from v1.5.0 of the exporter, OS_COMPUTE_API_VERSION can be set
to configure the Nova API version to be used [1]. Microversion 2.1 can
be used to keep metrics unmodified from the previous exporter version
deployed by Kolla (v1.3.0).
Support it with prometheus_openstack_exporter_compute_api_version,
defaulting to using the latest version.
[1] https://github.com/openstack-exporter/openstack-exporter/pull/201
Change-Id: I7605a3f9f74effb29ecec3b28e4709fd5f7f8cd4