Improve Pcep Server

Improve Pcep Server

Some PCCs send a PcRequest message with Metrics and/or Bandwidth set
with value equal to 0 which result to a NO_PATH when calling path
computation. Indeed, it is impossible to determine a path with a metric
less or equal to 0.

This patch set skip Metrics and/or Bandwidth equal to 0 when building
the list ofi constraints for the path computation algorithm.

Change-Id: I3b343e67fc35f3393d4ed136aa4224082823f066
Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>