fix(cgi-bin): add switch for is_member in set_membership

cgi-bin/set_membership

@@ -199,7 +199,7 @@ if (not $is_decode_json_success)
 
 my $host_uuid                = exists $request_body->{host_uuid} ? $request_body->{host_uuid} : $anvil->data->{switches}{'host-uuid'};
 # Defaults to join; will check whether host is already part of its anvil cluster.
-my $is_member                = exists $request_body->{is_member} ? $request_body->{is_member} : 1;
+my $is_member                = exists $request_body->{is_member} ? $request_body->{is_member} : $anvil->data->{switches}{'is-member'};
 my $host_uuid_variable_name  = "host UUID";
 
 if ($host_uuid)