Create Ceph user inplace of default client.admin

[c3y1huang]

Describe your changes

Resonate to the upcoming authentication strengthening section in SES7 doc. Added instruction to create the client user instead of using the default admin.

Other changes:

• add external links to instructions.
• replace placeholders with samples.

Related Issues / Projects

Relates to:

• https://github.com/SUSE/doc-caasp/issues/1005
• https://github.com/SUSE/doc-ses/blob/master/xml/hardening_measures_authentication.xml#L21-L75
• Add case to test Ceph client admin replacement