Export underlay source port as 0 when encapsulation type is MPLSoGRE.

In vrouter underlay source port calculation is done for each flow record irrespective of its encap type. Agent instead of exporting this value (given by vrouter) as-is, should send the value as 0 when encap type is MPLSoGRE

Add unit test case to verify this.

Change-Id: I2c582d83ba2dafdf65b351a374c8f87d4517464e
Closes-Bug: #1382355
(cherry picked from commit a3cd11361e5006dc630b0e51e91d9637c044aaaa)