Improve the Installation Guide: Compute agent for Telemetry section

1. Restructured content
2. Removed openstack-config commands
3. Replaced auth_host, auth_port and auth_protocol keys with identity_uri and auth_uri keys

Partial-bug: 1301034
Change-Id: Ifdc6068b5042811df3cb008b7e0a8f9c7ba4c51e
Implements: blueprint installation-guide-improvements
Merge "Improve the Installation Guide: Compute agent for Telemetry section"