Cinder and etcd are enabled by default and by default
cinder uses etcd as a distributed lock manager with
tooz as an intermediary. We see a lot of ToozConnectionErrors [1]
in the cinder logs when etcd is backed up [2] which results in
cinder operations timing out causing test failures, like
when a volume is not deleted within a given time.

This changes ETCD_USE_RAMDISK=True by default to try and
alleviate some of the pressure. An alternative is if we know
we're in a single-node job we could just not use a DLM for
Cinder.

[1] http://status.openstack.org/elastic-recheck/#1810526
[2] etcd[26824]: sync duration of 12.076762123s, expected less than 1s

Change-Id: I5f82aa40e9d84114e7b7b5cf19ec4942d6552490
Partial-Bug: #1810526