Update 'flyway.locations' appendix documentation

Replace `classpath:db/migrations` with `classpath:db/migrations`.

Fixes gh-2063
Add LoggingApplicationListener constants

Extract some of the common property keys as constants.

Fixes gh-2068
Merge branch '1.1.x'

Conflicts:
	spring-boot/src/main/java/org/springframework/boot/logging/LoggingApplicationListener.java