Fix traceback when ipsilon-tools-ipa is not installed
    
        # /usr/sbin/ipsilon-server-install
        'ipa'
        Traceback (most recent call last):
          File "/usr/sbin/ipsilon-server-install", line 426, in <module>
            opts = parse_args(fplugins)
          File "/usr/sbin/ipsilon-server-install", line 413, in parse_args
            if len(args['lm_order']) == 0 and args['ipa'] != 'yes':
        KeyError: 'ipa'
    
    The problem is that ipa get added as a argument to ipsilon-server-install,
    which can be absent (at least on RHEL 7).
    
    Signed-off-by: Michael Scherer <misc@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>