This commit addresses a few shortcomings in the etcd service:
* Adding or removing etcd nodes required manual intervention.
* The etcd service would have brief outages during upgrades or
reconfigures because restarts weren't always serialised.
This makes the etcd service follow a similar pattern to mariadb:
* There is now a distiction between bootstrapping the cluster
and adding / removing another member.
* This more closely follows etcd's upstream bootstrapping
guidelines.
* The etcd role now serialises restarts internally so the
kolla_serial pattern is no longer appropriate (or necessary).
This does not remove the need for manual intervention in all
failure modes: the documentation has been updated to address the
most common issues.
Note that there's repetition in the container specifications: this
is somewhat deliberate. In a future cleanup, it's intended to reduce
the duplication.
Change-Id: I39829ba0c5894f8e549f9b83b416e6db4fafd96f
ed3b27cc92