(#22383) Allow all boolean values for atboot

The previous restrictions placed on the atboot property for mount caused some
previously valid manifests to begin to fail. The problem stems from the fact
that atboot is only used on solaris, but can always be specified by users. It
used to be able to be nearly anything, but only yes and no meant anything on
solaris. This fixes the problem by expanding the allowed values to include
all of the "boolean" values as expressed in the
Puppet::[Parameter|Property]::Boolean type and using a munge method to
convert the boolean to the correct yes or no value.