selftests: mlxsw: vxlan: Make the test more flexible for future use

vxlan.sh cases are dedicated to test VxLAN with IPv4 underlay.
The main changes to test IPv6 underlay are IP addresses and some flags.

Add variables to define all the values which supposed to be different
for IPv6 testing, set them to use the existing values by default.

The next patch will define the new added variables in a separated file,
so the same tests can be used for IPv6 also.

Rename some functions to include "ipv4", so the next patch will add
equivalent functions for IPv6.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>