Bug-2436: PCEP segment-routing - take F and S flag into account

-if F flag is set, NAI value is ommited
-if S flag is set, SID value is ommited

Change-Id: Ia92680c77af0a6f8b1406de4723e9e9174479a4c
Signed-off-by: Milos Fabian <milfabia@cisco.com>
(cherry picked from commit 87e629b4ad88887bc356700ff15d261dbe405d74)