Bug-2064: Implementation of RFC5492 Error handling.

-Unsupported Capability - subcode 7 for Open message error.
-Unsupported Capability MAY be used by BGP speaker to complain that peer does not support required capability.
The NOTIFICATION message MUST contain list of such capabilities in Data field.
-Unsupported Capability MUST NOT be used when a BGP speaker receives unrecognized or unsupported capability.
Such capabilities MUST be ignored.

The BGP speaker is prepared to receive
an OPEN message that contains one or multiple Capabilities Optional
Parameters, each of which contains one or more capabilities TLVs.

Change-Id: I09f12011d90dc77a6a4cc4b3049ff7eba71dc443
Signed-off-by: Milos Fabian <milfabia@cisco.com>