use extras to install optional dependencies

[Steve Martinelli]

we should take advantage of pbr's support for optional dependencies and install memcache, pycrypto and messaging with this mechanism. this will be consistent with keystone server support.

[Steve Martinelli (stevemar)]

patch: https://review.openstack.org/#/c/274400/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/322501

[OpenStack Infra (hudson-openstack)]

Change abandoned by Steve Martinelli (<email address hidden>) on branch: master
Review: https://review.openstack.org/322501
Reason: based on the outcome from the keystone meeting on 06/28/16 we should abandon this in favor or using oslo.cache (and having it be a hard requirement)

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/274400
Committed: https://git.openstack.org/cgit/openstack/keystonemiddleware/commit/?id=ef29dfce898af273fef7b4798920cc981644de62
Submitter: Jenkins
Branch: master

commit ef29dfce898af273fef7b4798920cc981644de62
Author: Steve Martinelli <email address hidden>
Date: Sun Jan 31 00:48:01 2016 -0500

    Use extras for oslo.messaging dependency

    Install the oslo.messaging optional dependency by doing:

      keystonemiddleware[audit_notifications]

    pbr documentation:

      http://docs.openstack.org/developer/pbr/#environment-markers

    Partial-Bug: 1540115

    Change-Id: I59eea1f7eb0e770ac4fe30211eff49ae76fb2550