Restore get_available_resource method in docker driver

The get_available_resource method, a necessary public
method was marked as private and it snuck through
code review! We caught this with docker-ci, but
only after it was merged.

This patch makes get_available_resource a public
method again, removing the _ prefix.

This is a partial revert for change:
    58d7eeede1e2912ab250fd306c85ca1a05de2fbc

Closes-Bug: 1286457

Change-Id: I07cede8e1ebf606988aeec5bb4d65a9c5ebfe20b