Extract config server properties to Java bean

ConfigServerProperties is now used to store global properties
for the config server (like the path prefix and bootstrap flag)
under spring.cloud.config.server.* so git specific ones have
been pushed down to spring.cloud.config.server.git.* (similarly
for git->native).

At the same time introduce spring.cloud.config.server.defaultLabel
for the deafult git label (defaults to "master").

Fixes gh-28