Requiring that a bundle defines more than one OSD is unecessary

While the disk replacement rests are good for the OSD charm,
and it's useful to have more disks by default in that case,
requiring extra disk for all applications that tun the Ceph tests
is an unreasonable burden, requiring additional changed just
to accommodate the OSD charm tests.

This change updates to check if we have enough disks available
before running the disk replacement tests.

zaza/openstack/charm_tests/ceph/tests.py

@@ -563,13 +563,14 @@ class CephTest(test_utils.BaseCharmTest):
         self.assertEqual(results['osd-ids'], osd_id)
         zaza_model.run_on_unit(unit, 'partprobe')
 
-    def remove_one_osd(self, unit):
+    def remove_one_osd(self, unit, block_devs):
         """Remove one device from osd unit.
 
         :param unit: Unit name where the osd device is to be removed from.
         :type unit: str
+        :params block_devs: list of block devices on the scpecified unit
+        :type block_devs: list[str]
         """
-        block_devs = self.get_osd_devices_on_unit(unit)
         # Should have more than 1 OSDs to take one out and test.
         self.assertGreater(len(block_devs), 1)
 
@@ -593,10 +594,17 @@ class CephTest(test_utils.BaseCharmTest):
         # Remove one of the two disks.
         logging.info('Removing single disk from each OSD')
         for unit in osds:
-            device_info = self.remove_one_osd(unit)
+            block_devs = self.get_osd_devices_on_unit(unit)
+            if len(block_devs) < 2:
+                continue
+            device_info = self.remove_one_osd(unit, block_devs)
             block_dev = device_info['block-device']
             logging.info("Removing device %s from unit %s" % (block_dev, unit))
             osd_info[unit] = device_info
+        if not osd_info:
+            raise unittest.SkipTest(
+                'Skipping OSD replacement Test, no spare devices added')
+
         logging.debug('Removed OSD Info: {}'.format(osd_info))
         zaza_model.wait_for_application_states()