[SPARK-7261] [CORE] Change default log level to WARN in the REPL

1. Add `log4j-defaults-repl.properties` that has log level WARN.
2. When logging is initialized, check whether inside the REPL. If so, use `log4j-defaults-repl.properties`.
3. Print the following information if using `log4j-defaults-repl.properties`:
```
Using Spark's repl log4j profile: org/apache/spark/log4j-defaults-repl.properties
To adjust logging level use sc.setLogLevel("INFO")
```

Author: zsxwing <zsxwing@gmail.com>

Closes #6734 from zsxwing/log4j-repl and squashes the following commits:

3835eff [zsxwing] Change default log level to WARN in the REPL