Added 'x-pkg verrel' for containers
    
    Users wanted the ability to use 'x-pkg verrel' to
    get relevant information about their container images.
    
    Method parses values in Dockerfile. There are two rules
    for parsing values from Dockerfile correctly.
    
    (1) <repo_name>_VERSION supercedes VERSION
    (2) If there are no VERSION values or no RELEASE value,
    	'verrel' fails on Dockerfile.
    
    + added changes suggested by @onosek
    
    Fixes: #547
    Jira: RHELCMP-4751
    Signed-off-by: jkunstle <jkunstle@redhat.com>