expose --secret option in radiusproxy-* commands

Option `--secret` was hidden from radiusproxy CLI preventing setting a secret
on existing server or searching by secret. Since thin client implementation it
was also not recognized by the interactive prompt code in CLI frontend since
it never got there.

https://fedorahosted.org/freeipa/ticket/6078

Reviewed-By: Jan Cholasta <jcholast@redhat.com>

ipaserver/plugins/radiusproxy.py

@@ -126,7 +126,6 @@ class radiusproxy(LDAPObject):
             label=_('Secret'),
             doc=_('The secret used to encrypt data'),
             confirm=True,
-            flags=['no_option'],
         ),
         Int('ipatokenradiustimeout?',
             cli_name='timeout',