[SPARK-1896] Respect spark.master (and --master) before MASTER in spark-shell

The hierarchy for configuring the Spark master in the shell is as follows:
```
MASTER > --master > spark.master (spark-defaults.conf)
```
This is inconsistent with the way we run normal applications, which is:
```
--master > spark.master (spark-defaults.conf) > MASTER
```

I was trying to run a shell locally on a standalone cluster launched through the ec2 scripts, which automatically set `MASTER` in spark-env.sh. It was surprising to me that `--master` didn't take effect, considering that this is the way we tell users to set their masters [here](http://people.apache.org/~pwendell/spark-1.0.0-rc7-docs/scala-programming-guide.html#initializing-spark).

Author: Andrew Or <andrewor14@gmail.com>

Closes #846 from andrewor14/shell-master and squashes the following commits:

2cb81c9 [Andrew Or] Respect spark.master before MASTER in REPL