Use _is_valid_ipv6 from oslo.utils

Remove _is_valid_ipv6 from oslo.vmware and replace it's usages
by usages of _is_valid_ipv6 from netutils, because this methods
implements the same functionality.

Change-Id: I79f3528d940558a19c4cdea419915a7ce4f843e3
Merge "Use _is_valid_ipv6 from oslo.utils"