OPENDJ-3130 Provide REST access to OpenDJ's configuration and monitoring

Implemented a new Rest2Ldap based endpoint "admin" which exposes two
paths:

* admin/monitor: a read-only endpoint providing a simple recursive
mapping to all entries exposed in cn=monitor. Unfortunately, cn=monitor
is schema-less so the attribute mappings are sub-optimal

* admin/config: a fully functional endpoint providing access to
cn=config. The mapping is driven from the configuration framework.

NOTE: the admin endpoint is only available in the embedded listener,
although we could make it available in the gateway if requested.