rhel: Add OVSREQUIRES to automatically bring up OpenFlow interface dependencies

rhel: Add OVSREQUIRES to automatically bring up OpenFlow interface dependencies

The use of OpenFlow controllers may require for a subset of
the network (typically the management network intrerface) to be
brought up before the openvswitch service is started.

The newly introduced key "OVSREQUIRES" allows to specify a
list of interfaces that need the be brought up before the
openvswitch service is autostarted. It is also possible to
build a chain of bridge dependencies.

	TYPE=OVSBridge
	OVSREQURIES="em1"
	[...]

A special UPPEDSTACK var that is passed along avoids getting
lost in dependency loops.

Signed-off-by: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>