Bug 915158 - Allow setting a marionette runner host at runtime.

This is in addition to the node-switch-over branch. Allow setting a marionette runner host at runtime.

Note: it default to marionette-b2gdesktop-host. It doesn't require the marionette-device-host I started playing with.

But it requires changes in PR #13497 to actually merge. It could be adapted to apply cleanly on master with a little work and then add the changes to the other branch.