Allow to run tests with PostgreSQL inside container
    
    This patch introduces a new environment variable in order to allow running
    tests with different database backend. This is useful particularly for running
    tests inside container. With this change, it is possible run tests in following
    combinations:
    
    * Python 2 and SQLite
    * Python 3 and SQLite
    * Python 2 and PostgreSQL
    * Python 3 and PostgreSQL
    
    Package python-psycopg2 is installed in both Dockerfile-tests and
    Dockerfile-tests-py3 in case of running tests with PostgreSQL.
    
    A new script contrib/run-unittests.sh is added to make it easy to run tests. An
    example:
    
        contrib/run-unittests.sh --py3 --with-pgsql
    
    that runs tests with Python 3 and PostgreSQL.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>