allow multiple init procs on host configurations

This switches the host configuation to allow defining multiple
init procs.  Now the `init` method just pushes the proc onto the
configs list of init procs.  On host data init, the host data will
call each init proc in the list.

This is mostly about flexibility and extensibility.  Now multiple
host mixins can each define their own init blocks and add in their
own init behavior, etc.

Closes #83.

@jcredding ready for review.