Add option to place etcd data dir on RAM disk
Add option to place etcd data dir on RAM disk

Seems like for etcd-heavy services like Kubernetes, the fsync
performance of gate VM's are too low [1]. This commit implements an
option to put etcd data directory on RAM disk (tmpfs) to work this
around.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2019-January/001849.html

Change-Id: I5a17099cb9d6941b1a009dc82daefd2c7946d892

dtroyer/devstack,dtroyer/devstack,nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack