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
Merge "Ensure is_backend_avail() doesn't leave open connections"