Removing duplicated availability zones from tests

This patch remove duplicated availability zones when retrieving them
from service list.

Closes-Bug: #1875180
Change-Id: I0ee61c3197547201f10340d388baf5e076494be5
diff --git a/manila_tempest_tests/tests/api/base.py b/manila_tempest_tests/tests/api/base.py
index 8c18a4b..bdd48f8 100644
--- a/manila_tempest_tests/tests/api/base.py
+++ b/manila_tempest_tests/tests/api/base.py
@@ -711,7 +711,7 @@
                  service['binary'] == 'manila-share' and
                  service['state'] == 'up' and
                  re.search(backends, service['host'])]
-        return zones
+        return list(set(zones))
 
     @classmethod
     def get_pools_matching_share_type(cls, share_type, client=None):