devstack vagrantfile: mount "themes" from host

This mounts the "themes" directory on the host.  This will enable people
who have configured "stanford-style" themes to check out their themes on
the host and have it useable in the devstack.

Two things are a bit odd about this that are worth highlighting.

1. For people not using Stanford-style theming, which are most
   developers and certainly everyone at edX.org, this is a wasted
   export.  It doesn't do any harm, but would it be confusing?  I
   think that's OK.

2. The provision steps checks out the other repos that you need
   (edx-platform, cs_comment_service, etc.).  We're not checking out
   the theme repo ourselves here, counting on the user to do so
   yourself later.  I know this isn't consistent, but I couldn't
   plumb through the theming stuff into the rest of devstack
   provision, at least not right now.
turning off serial for test
installing rabbit from a deb file
removing protocol
set gather_facts to true
adding serial back
Merge pull request #989 from edx/jarv/rabbit-from-deb

Jarv/rabbit from deb
check before installing rabbit
syntax
Merge pull request #990 from edx/jarv/do-not-reinstall-rabbitmq

check before installing rabbit
Refactor key handling.
Merge pull request #991 from edx/e0d/datadog-refactor

Refactor key handling.
mising stdout on var check
Merge pull request #992 from edx/jarv/do-not-reinstall-rabbitmq

mising stdout on var check
Add LTI stub forwarded port.
Merge pull request #995 from edx/oleg/add-lti-stub-port

Add LTI stub forwarded port.
ansible-roles repo has been renamed ansible-private
Merge pull request #996 from edx/derf/ansible-roles-rejiggering

roles within ansible-roles have been moved into a roles subdirectory.
Explicitly set ssh to insecure private key.  It is default but sometimes does not work and requires public key
Update AUTHORS