NM autopkgtests fail IPv6 validation for non-SLAAC cases

Only a single example:

======================================================================
FAIL: test_manual_ip6_raonly_pubaddr (__main__.ColdplugEthernet)
ethernet: manual connection, IPv6 with only RA, preferring public address
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/adt-run.onjRP6/build.gYt/network-manager-1.1.93/debian/tests/nm", line 631, in test_manual_ip6_raonly_pubaddr
    ip6_privacy=NetworkManager.SettingIP6ConfigPrivacy.PREFER_PUBLIC_ADDR)
  File "/tmp/adt-run.onjRP6/build.gYt/network-manager-1.1.93/debian/tests/network_test_base.py", line 351, in wrapped
    args[0].wrap_process(fn, *args, **kwargs)
  File "/tmp/adt-run.onjRP6/build.gYt/network-manager-1.1.93/debian/tests/network_test_base.py", line 255, in wrap_process
    self.fail(f.read())
AssertionError: Traceback (most recent call last):
  File "/tmp/adt-run.onjRP6/build.gYt/network-manager-1.1.93/debian/tests/network_test_base.py", line 244, in wrap_process
    fn(*args, **kwargs)
  File "/tmp/adt-run.onjRP6/build.gYt/network-manager-1.1.93/debian/tests/nm", line 701, in do_test
    self.check_low_level_config(self.dev_e_client, ipv6_mode, ip6_privacy)
  File "/tmp/adt-run.onjRP6/build.gYt/network-manager-1.1.93/debian/tests/nm", line 372, in check_low_level_config
    self.assert_iface_up(iface, expected_ip_a, unexpected_ip_a)
  File "/tmp/adt-run.onjRP6/build.gYt/network-manager-1.1.93/debian/tests/nm", line 211, in assert_iface_up
    self.assertRegex(out, r)
  File "/usr/lib/python3.5/unittest/case.py", line 1289, in assertRegex
    raise self.failureException(msg)
AssertionError: Regex didn't match: 'inet6 2600::[0-9a-f:]*ff:fe[0-9a-f:]+/64 scope global (?:tentative )?(?:mngtmpaddr )?(?:noprefixroute )?dynamic' not found in '9: eth42@veth42: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000\n link/ether a2:db:d3:21:09:7b brd ff:ff:ff:ff:ff:ff\n inet6 2600::16/128 scope global dynamic \n valid_lft 3590sec preferred_lft 3590sec\n inet6 2600::345d:fa96:28ff:40df/64 scope global temporary dynamic \n valid_lft 3589sec preferred_lft 3589sec\n inet6 2600::ab06:f5a:2116:306/64 scope global mngtmpaddr noprefixroute dynamic \n valid_lft 3590sec preferred_lft 3590sec\n inet6 fe80::70a2:f5ce:e46c:42bd/64 scope link \n valid_lft forever preferred_lft forever\n'

See https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-xenial/xenial/amd64/n/network-manager/20160409_002439@/log.gz; failing autopkgtests for NM 1.1.93-0ubuntu1 upload to Xenial.

NM autopkgtests need to account for the new use of stable-privacy as a default addrgenmode for new connections.