debian: Make dependencies on openvswitch packages specify exact version.

NOX packages depend on a particular version of openvswitch-pki, which
depends on openvswitch-common without specifying a version.  This meant
that the installed versions of openvswitch-pki and openvswitch-common
could easily get out of sync.  This commit makes all of the dependencies
among openvswitch packages specify an explicit version, which should fix
this problem.

CC: Dan Wendlandt <dan@nicira.com>