Deprecate RestAuthConfig#allowed? in favor of #check_authorization

#allowed? was a poorly named method since it isn't actually a predicate
method. Instead of returning a boolean, this methods throws an
exception when the access is denied (in order to keep the full context
of what ACE triggered the deny).

Given that #allowed? was overriding the behavior from AuthConfig, we
leave a version of #allowed? in place that will issue a deprecation
warning before delegating to #check_authorization. Once support for
XML-RPC agents is removed from the master, we will be able to remove
this delegation, since there should no longer be a reason for a
distinction between AuthConfig and RestAuthConfig.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>
Signed-off-by: Jacob Helwig <jacob@puppetlabs.com>