cephadm: add unwrap_ipv6 helper method

cephadm: add unwrap_ipv6 helper method

When we pass in a mon-ip that is ipv6 we want it wrapped, this is so it
can be properly inserted into the mon_addrv address.
But there are times we need to unwrap it to test it's a valid ipv6
address.

This patch adds a helper method `unwrap_ipv6` which takes a string and
returns it either unwrapped or as it is, so it's harmless to other types
of ips.

This allows us to check a wrapped ipv6 base_ip with the networks on the
host.

Signed-off-by: Matthew Oliver <moliver@suse.com>