PGW_Tests: refactor/improve several parameters to accomodate for different GTPv2C interfaces

So far we were only testing s5/s8 interface, but we'll want to test s2b
soon.
This commit is a mixture of refactorings and code improvements as a step
towards testing S2b interface.

Change-Id: I22b3e18d02ca828e2ea43bde2e0a602db236cf50
2 files changed