dpif-netdev: add subtable-lookup-prio-get command.

dpif-netdev: add subtable-lookup-prio-get command.

This commit adds a new command, "dpif-netdev/subtable-lookup-prio-get"
which prints the available subtable lookup functions in this OVS binary.
Example output from the command:

Available lookup functions (priority : name)
        0 : autovalidator
        1 : generic

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>