mockbuild: escape rpm command under mock
    
    Arguments of the 'rpm' part after 'mock --shell' needs to be escaped.
    Example:
    
      mock -r xxx --chroot ... --shell rpm --define '_sourcedir xxx' ...
    
    JIRA: RHELCMP-10150
    Resolves: rhbz#2130349
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>