Cannot join on IPv6 address

[KaiSforza]

Summary

Using an IPv6 address as the join connection IP address causes the address to fail the get_valid_connection_parts function, since it spljits on :, which are separators for IPv6 addresses. There is already an ipaddress.ip_address call that should validate everything, I don't think we need the len(master_ep) != 2, and should probably do a .rsplit(":", 1) here instead.

What Should Happen Instead?

I should be able to type

microk8s join add0:a:1:25000/token/fp
# or
microk8s join [add0:a:1]:25000/token/fp

and join the master at add0:a:1.

Reproduction Steps

1. Create a microk8s node with an IPv6 address
2. Type microk8s add-node
3. Copy one of the lines with the IPv6 address on it
4. See Invalid connection: Expected format: <master_IP>:<master_PORT>/<token>[/<fingerprint>]

Can you suggest a fix?

Remove the check for if len(master_ep) != 2 here and edit https://github.com/canonical/microk8s/blob/master/scripts/wrappers/common/cluster/utils.py#L573 to do an ...rsplit(":", 1) to just split on the one side and get the IP separately from the port.

Probably also need to add a strip("[]") to remove the brackets from an IPv6 address.