Incorrect message when KRA already installed
    
    When trying to install a second time KRA, in case domain-level=0 the error lessage is not correct. It mentions :
    
     "ipa-kra-install: error: A replica file is required."
    
    Note that this behavior is not observed if domain-level=1
    The subject of the fix consist in checking that KRA is not already installed before going ahead in the installation process.
    
    Tests done: I have made the following  tests in bot domain-level=0 and domain-level=1 :
     - Install KRA (check it is correctly installed),
     - Install KRA a second time (check that the correct error message is raised)
     - uninstall KRA (check that it is correctly uninstalled)
     - Install KRA again (check that it is correctly installed)
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>