network: vxlan: add new test

Virtual eXtensible Local Area Network (VXLAN) provides L2 networks
over existed L3 networks. It is using UDP (port 8472) to encapsulate
data packets. More information:
http://tools.ietf.org/html/draft-mahalingam-dutt-dcops-vxlan-08

Test cases:
1. Local test, check if we can create 5000 VXLAN interfaces.
2. Local test, check if we can create and then delete VXLAN
interface 5000 times.
3. Requires remote host, test will setup virtual sub-net
192.168.124.*/24 between two hosts, then will compare
TCP performance with and without VXLAN (tcp_fastopen test).
4. The same as above but must fail, because VXLAN allows
to communicate only within the same VXLAN segment.

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
network: vxlan: add tests to runtest/network_stress

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>