BUG-636: Add configuration option to PCEP

This patch adds the ability to configure a RFC2385 key on a PCEP server
session. The configuration is accepted only if the runtime supports it,
e.g. PCEPTopologyProviderModule checks for presence before ever
attempting to use it.

Change-Id: I349cdef08655ec56ba2ed413f67ab187b0a9fc0c
Signed-off-by: Robert Varga <rovarga@cisco.com>