Output reg support for nicira experimenter fields

Output reg support for nicira experimenter fields

Implement NXAST_OUTPUT_REG2 which supports experimenter fields vs the
existing NXAST_OUTPUT_REG that does not. The openflowplugin API will
still expose a single output reg action, but this will be converted to
openflowjava output reg2 action if the source choice is a experimenter
field or to the existing output reg action otherwise.

From [1], this is the structure of the output reg2 action:

struct nx_action_output_reg2 {
    ovs_be16 type;            // OFPAT_EXPERIMENTER = 0xFFFF
    ovs_be16 len;             // 24.
    ovs_be32 vendor;          // NX_VENDOR_ID = 0x00002320
    ovs_be16 subtype;         // NXAST_OUTPUT_REG2 = 32

    ovs_be16 ofs_nbits;       // (ofs << 6) | (n_bits - 1).
    ovs_be16 max_len;         // Max length to send to controller.
    /* Followed by:
     * - 'src', as an OXM/NXM header (either 4 or 8 bytes).
     * - Enough 0-bytes to pad the action out to 24 bytes. */
};

[1] https://github.com/openvswitch/ovs/blob/master/lib/ofp-actions.c

Change-Id: I6865ea6b8b3191895f2c548e7acf6707d94402e0
JIRA: OPNFLWPLUG-1017
Signed-off-by: Jaime Caamaño Ruiz <jcaamano@suse.com>