Make compute metadata interface consistent with object store

[Everett Toews]

Make metadata handling consistent in Compute

Make metadata handling consistent across all Compute resources.
The consistent methods are get_*_metdata, set_*_metadata, and
delete_*_metadata which is similar to the __get__, __set__, and
__delete__ methods of a descriptor or the __getitem__, __setitem__,
and __delitem__ of a MutableMapping so it should be fairly natural
for Python users.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/285840

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/285840
Committed: https://git.openstack.org/cgit/openstack/python-openstacksdk/commit/?id=1f6990edbf31096f7d0c82dabb9a16c1b99af575
Submitter: Jenkins
Branch: master

commit 1f6990edbf31096f7d0c82dabb9a16c1b99af575
Author: Everett Toews <email address hidden>
Date: Sun Feb 28 14:07:00 2016 -0600

    Make metadata handling consistent in Compute

    Make metadata handling consistent across all Compute resources.
    The consistent methods are get_*_metdata, set_*_metadata, and
    delete_*_metadata which is similar to the __get__, __set__, and
    __delete__ methods of a descriptor or the __getitem__, __setitem__,
    and __delitem__ of a MutableMapping so it should be fairly natural
    for Python users.

    Change-Id: I977e8741bdd755e8af8c0f3c4e16e38cfba79bd5
    Closes-Bug: #1546156