Add placeholder resolution to @PropertySource processing

Previously the core Spring processing of @PropertySource would
resolve placeholders in the location attribute, but the pre-loading
of the property source by Spring Boot didn't do that. Now implemented
using Environment.resolvePlaceholders() (N.B. at a time when the only
Environment entries available are system properties and OS env vars).

E.g.

	@Configuration
	@PropertySource("classpath:/${source.location}.properties")
	protected static class WithPropertySourcePlaceholders {
           ...
	}