Use github for swift3.

swift3 is not on OpenStack infra (yet) use the github url instead.

Closes-Bug: #1275923
Change-Id: I0cc393f93b65dcf8642b3a35925eb9eba3c2e1eb
Merge "Use github for swift3."
Merge branch 'master' of https://github.com/openstack-dev/devstack