netinfo: fix defaultdict regression

netinfo.get is supposed to retun a map containing the keys:
- bondings
- bridges
- networks
- nics
- vlans

The change http://gerrit.ovirt.org/#/c/21652/5/lib/vdsm/netinfo.py
replaced the dictionary initialization to 'networks': {} (and
assignment of the other keys to a dict that could be empty) with a
defaultdict.

Unfortunately since not always are there gonna be elements for each
of the keys that get() is supposed to return, get() could end up
reporting only 'nics', creating all sorts of havoc in caps.py and
any get() consumer. (It failed the unit tests on machines without
vdsm networks defined).

Change-Id: I5c59eea34f7ad7f28428292f064698d1d04cbdb5
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21850
Reviewed-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>