* 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
bbb9f3164c