Set watcher pause on all nodes

Instead of just setting the watcher pause file, which isn't replicated,
RPC is used to set it on all nodes (where possible). This means that
after an outage of the master node and a following master-failover, the
watcher will still be paused.

The RPC is attempted on all nodes. Errors for nodes marked offline are
ignored. All other errors are returned in the form of an exception
(there is no other option given the LUXI API).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>