Mark test_volume_boot_pattern as slow

Based on the average test times taken from the last 300 runs
of the tempest-full job [1] the test_volume_boot_pattern
scenario test is the slowest test that is not marked slow at
208 seconds.

This change marks that test as slow which means it will still
be run in the tempest-slow job.

[1] paste.openstack.org/show/745114/

Change-Id: Ibc5e5ecaa997744aecea8aff07e7a7e06178f5f3
Related-Bug: #1783405

tempest/scenario/test_volume_boot_pattern.py

@@ -76,6 +76,7 @@ class TestVolumeBootPattern(manager.EncryptionScenarioTest):
         self.snapshots_client.wait_for_resource_deletion(snapshot_id)
 
     @decorators.idempotent_id('557cd2c2-4eb8-4dce-98be-f86765ff311b')
+    @decorators.attr(type='slow')
     # Note: This test is being skipped based on 'public_network_id'.
     # It is being used in create_floating_ip() method which gets called
     # from get_server_ip() method