Add an option to specify a custom JDBC driver in the installer

Sometimes when installing Confluence, a custom JDBC driver may be needed. This could be because of various reasons, including, but not limited to:

1. An official driver is not distributed with Confluence, such as for Oracle
2. A customisation is required in the JDBC driver

Currently the installer won't prompt for this, and will immediately start Confluence without prompting, so it needs to be stopped first, and then manually have a JDBC driver installed before starting again, and continuing the install.

If there is a prompt to install this driver during the install and upgrade steps, this will save on the effort of restarting, and also gives an opportunity to make the administrator aware that they need such a driver, if their configuration needs it.