Bug 1890 - Fixing NX-Multipath action plugin

1. Setting right type for Dst as DstChoice
2. Updating convertor class to do right translation between OFJava and SAL
3. Updating ActionUtil call that uses the NX-Multipath action

Change-Id: Id226bab612cd3fa0c1243a00955e5a187fd7706a
Signed-off-by: Srini Seetharaman <srini.seetharaman@gmail.com>