Explicitly manage extra dependencies

Some storage driver needs additional requirements.

Change-Id: I7d4913f67658fc49894be4e095df24f86f4adbed

setup.cfg

@@ -92,6 +92,14 @@ zaqar.extraspec.tasks =
 oslo.policy.policies =
     zaqar = zaqar.common.policies:list_rules
 
+[extras]
+mongodb =
+    pymongo>=3.6.0 # Apache-2.0
+redis =
+    redis>=3.0.0 # MIT
+mysql =
+    PyMySQL>=0.8.0 # MIT License
+
 [nosetests]
 where=zaqar/tests
 verbosity=2
@@ -103,11 +111,3 @@ cover-html = true
 cover-erase = true
 cover-inclusive = true
 
-; Disabled: Causes a bug in testtools to manifest.
-; Trigger: self.assertX(condition), where condition == False.
-;
-; In "testtools/testresult/real.py" the traceback is set to
-; None in _details_to_exc_info(), but the inspect_traceback()
-; method in nose/inspector.py requires a traceback-like object.
-;
-; detailed-errors = 1