Migrate to pulse guardian (bug 1016117)

For bug 1016117.

This PR include a commit that moves from using the public/public user on Pulse which mcote is trying to migrate everybody away from... Instead someone must create `treeherder-staging` and `treeherder` users using pulse guardian `pulse.mozilla.org`, see bug 1082801.
can you put the disconnect() call in a `finally` clause?