python: Avoid star imports
    
    Star Imports (from module import *) are very convenient but flake8 is
    not smart enough to tell which names are imported. Replace the star
    imports with specific imports.
    
    This changes fixes these flake8 errors:
    
        5     F403 'from .constants import *' used; unable to detect undefined names
        44    F405 'MiB' may be undefined, or defined from star imports: .constants, .units
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>