Added Trove (database) version API tests

Added a new file "test_versions.py" to verify the current
version of the Database API.  Required supporting
functions are added in a new client file "versions_client.py"
under the JSON interface.
Modified api/base.py, etc/tempest.conf.sample, clients.py and
config.py files

Partially implements blueprint: trove-tempest

Change-Id: I3dbe4e40b8b2a1ec3c69573dd40c3c8a643d73d6
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 4a567e7..a1abe7f 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -442,6 +442,10 @@
 # value)
 #db_flavor_ref=1
 
+# Current database version to use in database tests. (string
+# value)
+#db_current_version=v1.0
+
 
 [debug]