Make a lazy AuthenticationManager if we think it's already configured

Instead of just blindly creating the default authentication manager, after
thic change we count the beans of type GlobalAuthenticationManagerConfigurer
and assume that if we detect more than we expect (one from Boot and one from
Spring Security) then the user is telling us they want to configure the
AuthenticationManager themselves.

Fixes gh-1801
Polish
Merge remote-tracking branch 'springsource/1.1.x'

Conflicts:
	spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/security/SecurityAutoConfigurationTests.java