Just some minor formatting and wording updates

Change-Id: Ia42944512f8d14a1993bd4ae3d09f0f2ab431322

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

When running without that option, the list of servers might be empty,
resulting in leftover instances on the host that is about to be
migrated.

Change-Id: I85915f0f86655f3a1eb5f54911d24e4e0e75f340

Forward-ported from Train commit
I19c7f05b538a7abc9253194bf041c037b1998378.

Change-Id: If07b84e0bbdcb7da8dbef87cc8826987f1d11cf8