client: wait for optional leases, just don't fail

When waiting for a group-optional lease pair (dhcp4 or dhcp6) and
one arrives, wait for the second one until timeout and recheck the
results then (bnc#876848,bnc#879127).
ifdown: do not try to fire nanny, it is not hired

We do not install nanny policies by default in ifup, so it is
not needed to try to delete them in ifdown. There is currently
no nanny started in inst-sys and the attempt is causing ifdown
to run into timeouts (bnc#876848,bnc#879127).
Merge pull request #257 from mtomaschewski/dhcp4or6

fixes for dhcp4,dhcp6 and ifdown/ifreload issues (bnc#876848,bnc#879127)