tests: Add support for automatically running Ryu tests against OVS.

tests: Add support for automatically running Ryu tests against OVS.

The Ryu controller comes with an extensive library of OpenFlow tests, but
it doesn't seem so easy to me to run all of them against a development
version of Open vSwitch.  This commit introduces a Makefile target so that
one can run all the Ryu tests with a simple "make check-ryu".

This commit adds documentation for the new target to INSTALL.  It also
moves the documentation for the "check-oftest" target and the
--enable-coverage configure option into INSTALL.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Reviewed-by: Simon Horman <horms@verge.net.au>