dhcp: add support for inet + inet6 dhcp on same interface

dhcp: add support for inet + inet6 dhcp on same interface

Ticket: CM-12370
Reviewed By: Roopa, Kanna, Scott E
Testing Done:

This patch also fixes a problem where dhcp6 used to create lease file with
a trailing whitespace. dhcp6 operation were also sometimes using the wrong
pid file. I added some code in the debian.postinst script to correctly
rename these files if they exists when we install/update ifupdown2.

(cumulus-qa-infra/cl-tests/tests/smoke/testdhcp.py:Testdhcp_relay)
auto swp1
iface swp1 inet dhcp
      link-speed 10000
      link-duplex full
      link-autoneg off

auto swp1
iface swp1 inet6 dhcp

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>