treewide: simplify dhcp leasetime checking

Set dhcp leasetim to 43200 seconds for all interfaces independant
if dhcpv4 is enabled for an interface. Minimum allowed leasetime
is 60 seconds when parsing leastime configuration; this allows to
clean up the awkward leasetime checks on different places in the code

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>