doc/radosgw: Promptify commands and improve formatting in keystone.rst

doc/radosgw: Promptify commands and improve formatting in keystone.rst

Use blocks with bash privileged command prompt for CLI
command examples. Separate example command output to a
preformatted block. Previously a hard-coded prompt in
some place inconsistently while no prompts in others.

Indent multi-line CLI command examples consistently.

Use Title Case consistency in section titles, instead
of some capitalized only first letter of title text.

Use double-backtick inline code for syntax strings,
HTTP header names etc as seems common.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>