Mark network slow test as slow

We have observed that tempest-full jobs was time out
many times.

  http://status.openstack.org/elastic-recheck/#1783405

Based on below ethercalc, we are trying to check the slow
tests and mark them slow so that those tests will not run
as part of tempest-full job. There is other job tempest-slow
which will run these tests.

  https://ethercalc.openstack.org/dorupfz6s9qt

Compute slow tests have been marked slow in
- I2a0e154ba38c7407b41b7e986a0bf9b451c65cae

This commit mark network slow tests based on above ethercalc.

Change-Id: Ic2b3f5ea5b6885fe727a21810ddd9e17b779a1a0
Partial-Bug: #1783405

tempest/scenario/test_network_v6.py

@@ -280,6 +280,7 @@ class TestGettingAddress(manager.NetworkScenarioTest):
                                dualnet=True)
 
     @decorators.idempotent_id('9178ad42-10e4-47e9-8987-e02b170cc5cd')
+    @decorators.attr(type='slow')
     @utils.services('compute', 'network')
     def test_dualnet_multi_prefix_slaac(self):
         self._prepare_and_test(address6_mode='slaac', n_subnets6=2,