ipaclient/action_plugins/ipaclient_get_otp: Only require gssapi for keytab

gssapi is only needed for OTP if keytab is used. The common case with
password does not require gssapi.

This change also fixes the new ansible 2.8 failure if gssapi is not
installed on the controller. Ansible 2.8 seems to also transfer and load
action plugins to the node if they are not used.