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>

ifupdownaddons/modulebase.py

@@ -15,6 +15,7 @@ import shlex
 
 from ifupdown.utils import utils
 from ifupdown.iface import *
+import ifupdown.policymanager as policymanager
 #from ifupdownaddons.iproute2 import *
 #from ifupdownaddons.dhclient import *
 #from ifupdownaddons.bridgeutils import *
@@ -38,6 +39,11 @@ class moduleBase(object):
         self.CACHE = kargs.get('cache', False)
         self.CACHE_FLAGS = kargs.get('cacheflags', 0x0)
 
+        # vrfs are a global concept and a vrf context can be applicable
+        # to all global vrf commands. Get the default vrf-exec-cmd-prefix
+        # here so that all modules can use it
+        self.vrf_exec_cmd_prefix = policymanager.policymanager_api.get_module_globals('vrf', attr='vrf-exec-cmd-prefix')
+
     def log_warn(self, str, ifaceobj=None):
         """ log a warning if err str is not one of which we should ignore """
         if not self.ignore_error(str):