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