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.
I don't understand you choice here, why didn't you use a dict to hold the available exchanges?