subnet_is_ipv6() function does not work for types ipv6_dhcpv6-stateless|stateful

Bug Description

def subnet_is_ipv6(subnet):
"""Common helper for checking network_state subnets for ipv6."""
# 'static6' or 'dhcp6'
if subnet['type'].endswith('6'):
# This is a request for DHCPv6.
return True
elif subnet['type'] == 'static' and is_ipv6_addr(subnet.get('address')):
return True
return False

Function return false for ipv6_dhcpv6-stateless|stateful, the eni renderer does not add '6' to 'inet' so it's rendered like: 'iface iface0 inet auto|dhcp' not 'iface iface0 inet6 auto|dhcp'

This bug is fixed with commit fcc92ad1 to cloud-init on branch master.
To view that commit see the following URL:
https://git.launchpad.net/cloud-init/commit/?id=fcc92ad1

This bug is believed to be fixed in cloud-init in version 19.2-76. If this is still a problem for you, please make a comment and set the state back to New

Thank you.

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3476