ofp-actions: Add "ingress" and "egress" options to "sample" action.

ofp-actions: Add "ingress" and "egress" options to "sample" action.

Before Open vSwitch 2.5.90, IPFIX reports from Open vSwitch didn't include
whether the packet was ingressing or egressing the switch.  Starting in
OVS 2.5.90, this information was available but only accurate if the action
included a port number that indicated a tunnel.  Conflating these two does
not always make sense (not every packet involves a tunnel!), so this patch
makes it possible for the sample action to simply say whether it's for
ingress or egress.

This is difficult to test, since the "tests" directory of OVS does not have
a proper IPFIX listener.  This passes those tests, plus a couple that just
verify that the actions are properly parsed and formatted.  Benli did test
it end-to-end in a VMware use case.

Requested-by: Benli Ye <daniely@vmware.com>
Tested-by: Benli Ye <daniely@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Simon Horman <simon.horman@netronome.com>