New certificate management module.

There is a new certificate management module placed in the plugins
folder:

    plugins/modules/ipacert.py

The certificate module allows to request, revoke, release and retrieve
certificates for users, hosts and services.

Here is the documentation for the module:

    README-cert.md

New example playbooks have been added:

    playbooks/cert/cert-hold.yml
    playbooks/cert/cert-release.yml
    playbooks/cert/cert-request-host.yml
    playbooks/cert/cert-request-service.yml
    playbooks/cert/cert-request-user.yml
    playbooks/cert/cert-retrieve.yml
    playbooks/cert/cert-revoke.yml

New tests for the module can be found at:

    tests/cert/test_cert_client_context.yml
    tests/cert/test_cert_host.yml
    tests/cert/test_cert_service.yml
    tests/cert/test_cert_user.yml

The module has been co-authored by Sam Morris (@yrro) and Rafael
Guterres Jeffman (@rjeffman).