ML2 Cisco Nexus Cfg not persistent after reboot

When Openstack applies configuration to Nexus and then Nexus reboots
after having been in stable config state, OpenStack onfiguration will
be lost if the administrator had not done 'copy run start' on the
Nexus. The phrase 'stable config state' refers to all configurations
were successfully applied and there is no further activity.
This is unlike Issue 1399998 which resolved those configurations
inflight.

This fix will allow OpenStack user to select an parameter to
perform 'copy running-config startup-config' for each Nexus
configuration.  The default will be off to be consistent with
earlier behavior.   The enabling parameter to add to local.conf
file is as follows:

[ml2_cisco]
persistent_switch_config = True

Change-Id: If9aea79d81e8bbc35c30289e1fa193af1ecbf95e
Closes-bug:  #1405239