Description of problem:
https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/13/html-single/advanced_overcloud_customization/#sect-Enabling_Internal_SSLTLS_on_the_Overcloud

In the section: "16.2. Add the undercloud to IdM"

1. There are no details how to generate an OTP (one time password) for ipa_otp field.
We used:
/usr/libexec/novajoin-ipa-setup --principal {{ idm_principal }} --server {{ idm_server }} --realm {{ idm_realm }} --domain {{ idm_domain }} --hostname $(hostname -f) --precreate --password {{ idm_password }}

2. The field "generate_service_certificate" must be set to "true":
generate_service_certificate = true

3. The field "certificate_generation_ca" must be set to IPA.
The value "IPA" is obtained from running "getcert list-cas" command. Like bellow:
[root@undercloud ~]# getcert list-cas
CA 'SelfSign':
	is-default: no
	ca-type: INTERNAL:SELF
	next-serial-number: 01
CA 'IPA':
	is-default: no
	ca-type: EXTERNAL
	helper-location: /usr/libexec/certmonger/ipa-submit
CA 'certmaster':
	is-default: no
	ca-type: EXTERNAL
	helper-location: /usr/libexec/certmonger/certmaster-submit
CA 'dogtag-ipa-renew-agent':
	is-default: no
	ca-type: EXTERNAL
	helper-location: /usr/libexec/certmonger/dogtag-ipa-renew-agent-submit
CA 'local':
	is-default: no
	ca-type: EXTERNAL
	helper-location: /usr/libexec/certmonger/local-submit

4. The field "service_principal" mus be set to: nova/undercloud_FQDN

5. The following Kerbers principal names must be created:
- nova/undercloud_FQDN
- openstack/undercloud_FQDN
They can be created on IDM server with following commands:
ipa service-add nova/undercloud.FQDN
ipa service-add openstack/undercloud.FQDN

Please let me know if you have any questions.