Invalid CIDR definition message when one IP address is passed

[Nobuto Murata]

This is a nit picking. When an user passes one IP address mistakenly to the MetalLB question, they will get an error as follows.

$ sunbeam cluster bootstrap --role control --role compute --role storage
WARNING: Minimum system requirements (4 core CPU, 16 GB RAM) not met.
Management networks shared by hosts (CIDRs, separated by comma) (192.168.123.0/24):
MetalLB address allocation range (supports multiple ranges, comma separated) (10.20.21.10-10.20.21.20): 192.168.123.81
Invalid value for 'MetalLB address allocation range (supports multiple ranges, comma separated)': Invalid CIDR definition, must be in the form 'ip/mask'

However, the config accepts comma-separated ranges as well as CIDR notation so returning the following error is more appropriate.

> Invalid IP range, must be in the form of 'ip-ip' or 'cidr'

https://github.com/canonical/snap-openstack/blob/599e01aa263729d8f411241531bc424934b9ce05/sunbeam-python/sunbeam/commands/microk8s.py#L58-L73