Add official support to the usage of a different schema on Oracle

According to our official documentation, Oracle databases should always use the schema owned by the user JIRA uses to connect with the system, not allowing for a different schema to be specified for the usage of another user's schema.

Manually adding the schema-name property to dbconfig.xml leads to problems with indexes, which are created (by default) under the schema of the user JIRA is using for the connection. Then, at some point, JIRA tries to create those indexes under the schema its database is on, which fails with a ORA-00955 (object already exists) error.