Creating public vn to reuse among multiple tests

1.Create public vn as singleton and pass the same objects to all the test cases using the vn
2.Its done to avoid creating public vn multiple times with same ip from different test cases
3.Since public vn is created as singleton, same object will be passed to different test cases
4.Modified the mx and snat test cases for sanity
5.Fab changes done in a separate commit - public tenant name and the vn name is taken from sanity_pararms

Taken care of the comments

Change-Id: Ib0aaafebea7810f708d3376642fdf72f01aac0b3
Merge "Creating public vn to reuse among multiple tests"