Description of problem:
Tests from module tempest.scenario.test_network_v6.TestGettingAddress are failing on OSP-15 when RHEL8 is used as guest image.
At first glance it looks that RHEL 8 is using NM_SETTING_IP6_CONFIG_ADDR_GEN_MODE_STABLE_PRIVACY as addr-gen-mode and due to that "wrong" IPv6 address is configured on host.

Example of failure:

Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/tempest/common/utils/__init__.py", line 89, in wrapper
    return f(*func_args, **func_kwargs)
  File "/usr/lib/python3.6/site-packages/tempest/scenario/test_network_v6.py", line 273, in test_dualnet_dhcp6_stateless_from_os
    self._prepare_and_test(address6_mode='dhcpv6-stateless', dualnet=True)
  File "/usr/lib/python3.6/site-packages/tempest/scenario/test_network_v6.py", line 224, in _prepare_and_test
    (ip, srv['id'], ssh.exec_command("ip address")))
  File "/usr/lib/python3.6/site-packages/unittest2/case.py", line 693, in fail
    raise self.failureException(msg)
AssertionError: Address 2003::f816:3eff:fe54:9177 not configured for instance 786e183f-567a-4496-892a-702e5ed3a091, ip address output is
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc fq_codel state UP group default qlen 1000
    link/ether fa:16:3e:d9:29:f4 brd ff:ff:ff:ff:ff:ff
    inet 10.100.0.13/28 brd 10.100.0.15 scope global dynamic noprefixroute eth0
       valid_lft 86074sec preferred_lft 86074sec
    inet6 fe80::f816:3eff:fed9:29f4/64 scope link 
       valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc fq_codel state UP group default qlen 1000
    link/ether fa:16:3e:54:91:77 brd ff:ff:ff:ff:ff:ff
    inet6 2003::d105:c69d:11c1:3f79/64 scope global dynamic noprefixroute 
       valid_lft 86323sec preferred_lft 14323sec
    inet6 fe80::adcf:ec57:fed1:56c8/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever