docker regression with get_available_resource marked private

[Erica Windisch]

The docker driver no longer exposes a get_available_resource method as a regression introduced with change 58d7eeede1e2912ab250fd306c85ca1a05de2fbc (https://review.openstack.org/#/c/65837/)

This method was incorrectly renamed to be prefixed by a _ as a private method, although it must be public.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/77339

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/77339
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=831f22f786c066bd7ef7a03a7255eb61f9b84b05
Submitter: Jenkins
Branch: master

commit 831f22f786c066bd7ef7a03a7255eb61f9b84b05
Author: Eric Windisch <email address hidden>
Date: Sat Mar 1 01:39:40 2014 -0500

    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