vyatta-op-dhcp-server: tidy up output on "show dhcp server leases"

Tidy up the output of both "show dhcp server leases" and "show dhcp
server statistics".  This allows a little more room for those of us
that use a longer dhcp pool name and keeps the format style the same
across the two.

Bug #296 http://bugzilla.vyos.net/show_bug.cgi?id=296