README.md: Pair ovn-controller with ovn ipam
README.md: Pair ovn-controller with ovn ipam

ovn's code will constantly change. It is not feasible to build
a ipam server for every OVS commit and pushed to docker hub. So
in this documentation, we pair a ovn-controller commit with the
corresponding ipam version.

Signed-off-by: Gurucharan Shetty <c4089df073b4e0e1342dd7f47682381cc38caa5c@nicira.com>

shettyg/ovn-docker,shettyg/ovn-docker,shettyg/ovn-docker