Changes to mariadb charm

- Modify metadata to fit the requirements of CharmHub
- Improve the description of the actions
- Change SVG icon
- Improve the README

Credit goes to Tom Haddon, that actually did the commit in lp, in a repo
that is no longer maintained and will be soon removed

Change-Id: Ic011d6b4d9cf07e5ef02a0e1ecead3ffdfbc3a45
Signed-off-by: David Garcia <david.garcia@canonical.com>