[SSSD] [PATCH] Add callback when the ID provider switches from offline to online

[Stephen Gallagher]

Allow backends to set a callback in the be_ctx that should be
invoked when the ID provider goes online.

This can be used to perform regular maintenance tasks that are
valid only when going online.

Specifically, we can use this to perform a deferred kinit on behalf of 
the user when we go online. We can also use this to trigger a dynamic 
DNS update for the IPA provider.