Provide override for features detected during handshake

In order to support switches with buggy and/or partial implementations (f.e. OVS 2.0) it is necessary to have the option to override the feature set determined durig handshake.

Example: force the use of the meter feature (even if the switch does not report them as supported). Force not to use the meter feature (switch reports them as supported but we know it is buggy).