Enable akka WeaklyUp feature

Enable akka WeaklyUp feature

By enabling allow-weakly-up-members, akka will allow new nodes to join a
cluster if there are unreachable nodes. However, this only pertains to
new nodes that weren't previously in the cluster. Unfortunately it
doesn't pertain to node restarts where a node was in the cluster then
attempts to re-join with a new incarnation, which is what we really want.
Despite that, it will at least work for new nodes so I think it's worth
enabling. Akka might be further enhanced to broaden WeaklyUp to include
new incarnations (there's requests for that).

I also changed the ShardManager to handle MemberWeaklyUp events in
the same manner as MemberUp.

Change-Id: I5cf6c1967162b8a9bc6ffb59d34a50560699e4ca
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>