vagrant: pin devstack swift version

A patch recently committed to Swift
(034fae630cfd652093ef53164a7f9f43bde67336) completely broke ZeroCloud. I
don't know precisely the root cause; I found the bad commit by trail and
error.

This change pins devstack installations to a version of Swift just
before this "bad" patch.
Merge pull request #161 from larsbutler/vagrant-devstack-pin-swift

vagrant: pin devstack swift version