When Logback is the logging system, ensure that JBoss logging uses it

Previously, if Logback was being used as Boot's logging system, but
Log4J was also on the classpath before SLF4J and Logback, JBoss
Logging would use Log4J for its logging. This lead to warning messages
being produced as Log4J was not configured:

log4j:WARN No appenders could be found for logger (org.jboss.logging).
log4j:WARN Please initialize the log4j system properly.

This commit updates LogbackLoggingSystem to set the
org.jboss.logging.provider to "slf4j". This ensure that JBoss Logging
will use SLF4J and Logback as intended even when Log4J is also on the
classpath.

Closes gh-1928
Merge branch '1.1.x'

Conflicts:
	spring-boot/src/main/java/org/springframework/boot/logging/logback/LogbackLoggingSystem.java
	spring-boot/src/test/java/org/springframework/boot/logging/logback/LogbackLoggingSystemTests.java