Bug 498 - Fixed PCEP and BGP testtools

-added missing registration of extension parsers/serializers
-refactored ServiceLoaderBGPExtensionProviderContext - BGPExtensionProviderContext hold as a singleton

Change-Id: Idc5991a017c9bbc7550a1d4170f88f2086fd6416
Signed-off-by: Milos Fabian <milfabia@cisco.com>
Merge "Bug 498 - Fixed PCEP and BGP testtools"