Make the default RSA key size configurable
    
    There is still a compiled-in default (currently 2048)
    but this can be overridden in certmonger.conf with the
    rsa_key_size setting in the default section. This will
    allow users to increase the minimum size without changing
    the default behavior for others.
    
    https://pagure.io/certmonger/issue/211
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>