This commit restores the support of "spring.datasource.type". If this
property is set to an unsupported `DataSource` implementation, we now
properly create it again, rather than ignoring it because its value did
not match any value we support.

Since Spring Boot 1.4 now redirects `DataSource` implementation settings
to dedicated namespaces, the documentation has been updated to explicitly
mention how one can create such arrangement for an unsupported
implementation.

As a convenience, `DataSourceProperties` can initialize a
`DataSourceBuilder` based on its internal state, making it very easy for
anyone to create a `DataSource` from a `DataSourceProperties` instance.

Closes gh-6695