Change the way the tags and params are handled in rails

The rationale behind this patch is that it takes a lots of time
to let rails unserialize the ParamValue and ResourceTag object
on each compilation, just to throw them away the second after.
The idea is to fetch directly (and batched host per host) the
parameters and tags from the database and then returns them as
hash.
This allows the no-modification case to takes at least 2 times
less than before.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>