BUG-2299: allow PCEP extensions to be reconfigured

The trigger here is a change in the extensions cause the corresponding
context to be re-created from scratch, which leads to the topology
manager to being re-created. This patch implements instance reuse by
adding proper activator hooks.

Change-Id: I5f114b81ceeb26a3943afc4bd2f0c34b678ff029
Signed-off-by: Robert Varga <rovarga@cisco.com>