docker: return hypervisor_version as an int rather than a string

The compute_nodes.hypervisor_version type in the database is an integer
so the docker driver needs to return the hypervisor_version as an
integer rather than a string.

Also adds the missing unit test for get_available_resource.

Closes-Bug: #1253960

Change-Id: I4e2ac13f9203b54e71f1f5cd8e15311921dd85c5
Merge "docker: return hypervisor_version as an int rather than a string"