Updates to the 'Log in to the dashboard' section

Updated to reflect the following changes:
- New screenshots for the dashboard-admin and dashboard-project tabs
- Included info about Host Aggregates
- Changes to the Resource Usage tab
- Minor updates like 'Images & Snapshots' to 'Images'.
Partial-Bug: #1307860

Change-Id: I8deb81fc94fd115944bebd835dd17d39fbf43ffd
Merge "Updates to the 'Log in to the dashboard' section"