krb5 locator: fix IPv6 support
IPv6 addresses are added with surrounding '[' and ']' to the kdcinfo
file to be able to specify a port number properly. The Kerberos location
plugin didn't handle those entries properly.
Related to https://pagure.io/SSSD/sssd/issue/941
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>