Bugfix: Pass 'ephemeral' into the Registration object constructor.

The previous bugfix for the 'ephemeral' setting was setting _ephemeral
AFTER the znode had already been created... This meant that every node
was actually created as a persistent node, not ephemeral. Now we just
pass the ephemeral setting into the Registration object at construction
time.

Change-Id: Ie544cf800ecefb3ffbfb990615ab535055da7d51