ifreload causes unnecessary dhcp restart for eth0

Ticket: CM-5730
Reviewed By: roopa
Testing Done: tested change on Sean and Jason's lab setup

The dhcp restart on all interfaces using dhcp causes a reconnection
for NTP and in addition, a recent addition of rack management packages
(CM-5782) added ntp and a exit-hook script that also interrupted NTP
for a second time.   This patch will check for dhclient running (IPv4 and IPv6)
and not restart dhcp if it's already running.

addons/dhcp.py

@@ -23,6 +23,12 @@ class dhcp(moduleBase):
         self.ipcmd = None
 
     def _up(self, ifaceobj):
+        # if dhclient is already running do not stop and start it
+        if self.dhclientcmd.is_running(ifaceobj.name) or \
+               self.dhclientcmd.is_running6(ifaceobj.name):
+            self.logger.info('dhclient already running on %s.  Not restarting.' % \
+                             ifaceobj.name)
+            return
         try:
             if ifaceobj.addr_family == 'inet':
                 # First release any existing dhclient processes