Update standard-test-avocado role to support pip installation
    
    Avocado is not packaged for RHEL-8.0 Beta as it does not support
    python2 only rpms. This fix addresses the issue by installing
    avocado-framework using pip when using the standard-test-avocado
    role on RHEL-8.0
    
    Signed-off-by: Yash Mankad <ymankad@redhat.com>
    (cherry picked from commit c5e7cca62f9206582aa52fc01ae39397970e1e15)
    Signed-off-by: Yash Mankad <ymankad@redhat.com>