Ensure is_backend_avail() doesn't leave open connections

is_backend_avail() helper function calls _ensure_backenv_available()
method,which creates a SQLAlchemy engine and opens a test connection,
but doesn't call engine.dispose(). Depending on Python interpreter
version used, the connection in the pool may remain open for some
time (even though, we don't store a reference to an Engine instance).

Closes-Bug: #1393633

Co-Authored-By: Victor Sergeyev <vsergeyev@mirantis.com>
Change-Id: I0297af709ed18fed52308500c13054cb49adf401

oslo/db/sqlalchemy/utils.py

@@ -683,7 +683,8 @@ def is_backend_avail(backend, database, user=None, passwd=None):
                                      user=user,
                                      passwd=passwd)
     try:
-        provision.Backend._ensure_backend_available(connect_uri)
+        eng = provision.Backend._ensure_backend_available(connect_uri)
+        eng.dispose()
     except exception.BackendNotAvailable:
         return False
     else: