addons: dhcp: add vrf awareness

addons: dhcp: add vrf awareness

Ticket: CM-10292, CM-10282
Reviewed By: dsa, nikhil, julien
Testing Done: Tested dhcp config on an vrf slave

- Add support for policy module_globals variable
  "vrf-exec-cmd-prefix". It is read into per module
  self.vrf_exec_cmd_prefix variable

- If self.vrf_exec_cmd_prefix is present and interface is a
  vrf slave, use ifupdown2 will call such command in vrf
  context using:
    "%s %s %s" %(<vrf-exec-cmd-prefix>, <vrfname>, <cmd>)

- This also fixes calling of dhcp refresh when a dhcp slave is
  removed from a vrf

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>