Add support for kick-starting new datastores

The datastore type and version config settings are now
parameterized to allow any datastore.
The only missing information for new datastores
is the list of packages to install on the instance.

The datastore already supported by kick-start
(mysql, redis, percona) have been given default packages.
Packages for supported datastores can be overwritten by
setting the $PACKAGES variable before running cmd_set_datastore().

Change-Id: I72a715ea460e74e870a3385b753da9d8ad1411aa
co-authored-by: Kevin Conway <kevinjacobconway@gmail.com>