Restructured installation into submodules

Restructured installation into submodules

Because we are managing three separate applications which can be on
separate systems, splitting into submodules with shared paramaters and
other classes makes sense. Init.pp now throws an error if called
directly. Moved existing installation logic to mailman::core, which
handles the core service. Split yum repo resource into its own class
which is called by ::core and will be called by the other applications.

Also cleaned up unneeded mailman service user parameter; these are not
needed, the user is installed by the package and managed via the systemd
service. Created parameter for site_owner email address.

Fixed rspec tests; there is a known issue with using .merge!() on
options hashes. Changed semantics for confirming the absence of an
object on core__install_spec.rb (only looking for raise_error if we have
a specific message we want, otherwise using is_expected.to_not)

Change-Id: I60ef5e897d66574ee73be2768985b8915aa8d02f
Signed-off-by: Josh Farwell <jfarwell@linuxfoundation.org>