ip monitor: replace pairwise with grouper

pairwise takes an iterator or elements '123456789' and generates an
iterator that gives: ('1', '2'), ('2', '3'), ('3', '4'), ('4', '5')...

grouper takes an iterator or elements '123456789' and generates an
iterator that gives: [('1', '2'), ('3', '4'), ('5', '6'), ('7', '8'), ('9',
None)]

Since the data to parse from iproute2 commands is structured closer
to grouper, change to that.

Change-Id: Ie1c569cee85c2d4fd54cbd8e6d964c2291056c6c
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22092
Reviewed-by: Dan Kenigsberg <danken@redhat.com>