Jenkins unittests run in docker container
    
    Prepared dockerfiles for creating a testing environment in containers.
    Two containers are prepared. The first for normal Python3 and the
    second one runs in CentOS 7.
    A tox configuration is customized for supported versions of Python.
    
    JIRA: RHELCMP-4578
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>