add backend serveraliases to debian vhosts

deb.tfm.o (and friends) do not point to our host these days, but to a
CDN, thus, using deb.tfm.o as the vhost name is a tad wrong.

this change introduces two aliases for the vhost: deb-backend.tfm.o and
deb.${fqdn}, which in my test environment results in:

    ServerName deb.theforeman.org
    ServerAlias deb-backend.theforeman.org
    ServerAlias deb.repo-deb.tanso.example.com

This has the benefit that the right vhost is reachable without any
tricks, and will allow us to switch the CDN config to a "more correct"
naming scheme, later dropping deb.tfm.o from the vhost here totally.

(This has the side-benefit that deb.tfm LE requests go via the CDN and
only then hit our box, which is confusing to say the least and that
would stop)

puppet/modules/freight/manifests/user.pp

@@ -70,6 +70,7 @@
   include apache::mod::mime
 
   web::vhost { $vhost:
+    serveraliases => [ "${vhost}-backend.theforeman.org", "${vhost}.${facts['networking']['fqdn']}" ],
     docroot       => $webdir,
     docroot_owner => $user,
     docroot_group => $user,