dcnm_vrf: fix for #356, and for an undeploy case, simplify, more...

1. Fix case where previous commit in this PR broke undeploy.

2. Fix for issue #356

2. Update unit tests to align with changes in this commit

3. Some simplifications, including

- Add a method send_to_controller() to aggregate POST, PUT, DELETE verb handling.  This method calls dcnm_send() and then calls the response handler, etc.  This removes duplicated code throughout the module.

- Refactor vrf_lite handlng out of update_attach_params() and into new method update_attach_params_extension_values()

- Never noticed this, but it appears we don't have to use inspect() with the new logging system, except in cases where fail_json() is called.  Removed inspect() from all methods that do not call fail_json()

- New method is_border_switch() to remove this code from push_diff_attach() and for future consolidation into a shared library.

- Move dcnm_vrf_paths dictionary out of the class.  These endpoints will later be moved to common/api/ep/.

- in __init__(), add self.sn_ip, built from self.ip_sn.  There were several case where the module wanted a serial_number given an ip_address.  Added two methods that leverage self.sn_ip and self.ip_sn:

- self.serial_number_to_ip()
- self.ip_to_serial_number()

Replaced all instances where duplicated code was performing these functions.