Making changes to nxMultipathAction with MD-SAL

- Adding missing "dst" field in Yang nx-action-multipath-grouping
- Adding missing field to the ActionUtils.nxMultipathAction
- Adding setNwSrcAction and setNwDstAction to ActionUtils

Change-Id: Ie0aab975b17620a2a2b74b7a69acfd9251ddc6e4
Signed-off-by: Srini Seetharaman <srini.seetharaman@gmail.com>