Documentation of all predefined UIDs and GIDs in RHOSP

Issue

According to security compliance needs to understand the supported way to change the UID & GID of OpenStack service user.

A Red Hat subscription provides unlimited access to our knowledgebase, tools, and much more.

Here are the common uses of Markdown.

Code blocks

~~~
Code surrounded in tildes is easier to read
~~~

Links/URLs

[Red Hat Customer Portal](https://access.redhat.com)