netinfo: Determine bootproto from dhclient's lease files

Lease files are never deleted, just replaced so they do not
become too long. For DHCPv4 checking for an expiration time
of a lease was added, DHCPv6 leases use different flags.

Paths to lease files are predefined in lib/vdsm/netinfo.py
in _DHCLIENT_LEASES_GLOBS, more common ones are welcome.

Change-Id: I5fbc48a0adf5f40120a72ec2c4cc2fc80b7226b8
Bug-Url: https://bugzilla.redhat.com/987813
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23098
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>