Rename RT classes into misc::rt & role::rt

The current naming scheme for RT doesn't make much sense, as it embeds
"apache" in the name, due to the historical artifact of having the
previous installation under lighttpd. So, rename these to "rt" (as
puppet isn't always happy with dashes in class names, so let's avoid
"request-tracker").

Additionally, make the filenames of the classes consistent with the
naming of the classes themselves.

Finally, move a couple of includes to the role class where they belong
(namely: exim config, install_certificate, system_role).

Change-Id: I2184bfcf4ffc8bf8defb049472d4697af1670a91