Nxos vxlan vtep vni netcfg fix (#23620)

* Code cleanup
Removed 'add' method from CustomNetworkConfig.  It is identical
to the one inherited from NetworkConfig

* Removed unused CustomNetworkConfig import

* Replaced
```
def get_existing(module, args):
    existing = {}
    netcfg = get_config(module)
    config = netcfg.get_section(parents)
```
with
```
netcfg = CustomNetworkConfig(indent=2, contents=get_config(module))
```
get_config returns a string, not an object in 2.3.

* Removed non-functioning get_object method in CustomNetworkConfig in favor of the
inherited method.

Added child_objs property so that expand_selection would work.  The original
verion never worked correctly as it compared NetworkConfig obj's and str's.

* Removed ShellError method in favor or new load_config method.

* Removed ShellError method in favor or new load_config method.
fixes #20260

* nxos requires a "no" statement to change mcase group.  Corrected.
Corrected changed logic.

* Corrected deleted CustomNetworkConfig import

lib/ansible/modules/network/nxos/nxos_rollback.py

@@ -81,7 +81,6 @@ status:
 from ansible.module_utils.nxos import nxos_argument_spec, run_commands
 from ansible.module_utils.basic import AnsibleModule
 
-
 def checkpoint(filename, module):
     commands = ['terminal dont-ask', 'checkpoint file %s' % filename]
     run_commands(module, commands)