Added trove create via horizon

The trove chapter in the user guide did not have
a section on how to use horizon to create
a database instance.

This commit covers how to create a db instance
using horizon.

This commit is a partial implementation of the
blueprint.

Change-Id: I7419a59e1b1763298a89e85ffa845a327f87bcae
blueprint: user-guide-trove-chapter
Merge "Added trove create via horizon"