Run split on $disks if $disks is a string, not an array

The cephdeploy::osd function expects to receive an array containing
entries for the individual OSDs to be deployed. That doesn't work so
well with the Puppet dashboard because currently, the Puppet Dashboard
can only use strings as parameters. This commit adds a test for the
$disks variable; if it's a string, it will run split on the string and
create an array based on the split; if $disks is indeed an array, the
array will be passed on accordingly.
Ensure variable_prefix defaults to empty; add ceph_cluster_name

This commit ensures that the variable_prefix defaults to an empty value
or to the value of the configuration_variable_prefix parameter set by
the administrator in the site manifest or an ENC. It detaults to empty
to maintain compatibility with what the module previously did.

In addition, this commit adds a parameter for the ceph_cluster_name
parameter which was previously left out by accident.