ofproto-dpif: Add 'force-miss-model' configuration

ofproto-dpif: Add 'force-miss-model' configuration

This adds support for specifying flow miss handling behaviour at
runtime, through a new "other-config" option in the Open_vSwitch table.
This takes precedence over flow-eviction-threshold.

By default, the behaviour is the same as before. If force-miss-model is
set to 'with-facets', then flow miss handling will always result in the
creation of new facets and flow-eviction-threshold will be ignored. If
force-miss-model is set to 'without-facets', then flow miss handling will never
result in the creation of new facets (effectively the same as setting the
flow-eviction-threshold to 0, which is not currently configurable).

We intend to use this configuration option in the testsuite to force
particular code paths to be used, allowing us to improve test coverage.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Ben Pfaff <blp@nicira.com>