Add simple test to see if client is already configured

If this ever gets out of sync the user can always remove
/var/lib/ipa-client/sysrestore/*, they just need to understand the
implications.

One potential problem is with certmonger. If you install the client
and then re-install without uninstalling then the subsequent
certificate request by certmonger will fail because it will already
be tracking a certificate in /etc/pki/nssdb of the same nickname and
subject (the old cert).

ipa-client/ipa-install/ipa-client-install

@@ -440,6 +440,10 @@ def main():
     if options.uninstall:
         return uninstall(options)
 
+    if fstore.has_files():
+        print "IPA client is already configured on this system."
+        return 1
+
     cli_domain = None
     cli_server = None
     cli_realm = None