destroy kerberos cache before firefox in freeipa password change

In F25+, Firefox seems to do kerberos auth automatically, so if
we go to the FreeIPA admin URL while kerberos-authed as test4,
we are logged in right away as test4 - neat! But not what we
wanted. So let's kdestroy.

tests/freeipa_password_change_postinstall.pm

@@ -8,6 +8,8 @@ sub run {
     $self->console_login(user=>'root');
     # clear browser data so we don't go back to the 'admin' login
     assert_script_run 'rm -rf /root/.mozilla';
+    # clear kerberos ticket so we don't auto-auth as 'test4'
+    assert_script_run 'kdestroy -A';
     type_string "startx /usr/bin/firefox -width 1024 -height 768\n";
     assert_screen "firefox";
     start_webui("test1", "batterystaple");