Conntrack testing

Added tests for:
Table FLUSH
Table GET
Table DELETE with filter
Filter match

Use a simple UDP client to create flows into the conntrack for testing purpose
Each test will run in a separate network namespace so can run in parallel
Added kernel module dependencies into the travis file

Signed-off-by: Flavio Crisciani <flavio.crisciani@docker.com>

.travis.yml

@@ -4,5 +4,10 @@ before_script:
   - sudo sed -i -e 's/^Defaults\tsecure_path.*$//' /etc/sudoers
   # modprobe ip_gre or else the first gre device can't be deleted
   - sudo modprobe ip_gre
+  # modprobe nf_conntrack for the conntrack testing
+  - sudo modprobe nf_conntrack
+  - sudo modprobe nf_conntrack_netlink
+  - sudo modprobe nf_conntrack_ipv4
+  - sudo modprobe nf_conntrack_ipv6
 install:
   - go get github.com/vishvananda/netns