etcd: Add support for more scenarios

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