[SPARK-4348] [PySpark] [MLlib] rename random.py to rand.py

This PR rename random.py to rand.py to avoid the side affects of conflict with random module, but still keep the same interface as before.

```
>>> from pyspark.mllib.random import RandomRDDs
```

```
$ pydoc pyspark.mllib.random
Help on module random in pyspark.mllib:
NAME
    random - Python package for random data generation.

FILE
    /Users/davies/work/spark/python/pyspark/mllib/rand.py

CLASSES
    __builtin__.object
        pyspark.mllib.random.RandomRDDs

    class RandomRDDs(__builtin__.object)
     |  Generator methods for creating RDDs comprised of i.i.d samples from
     |  some distribution.
     |
     |  Static methods defined here:
     |
     |  normalRDD(sc, size, numPartitions=None, seed=None)
```

cc mengxr

reference link: http://xion.org.pl/2012/05/06/hacking-python-imports/

Author: Davies Liu <davies@databricks.com>

Closes #3216 from davies/random and squashes the following commits:

7ac4e8b [Davies Liu] rename random.py to rand.py

1 files changed, 0 insertions, 10 deletions

diff --git a/python/pyspark/__init__.py b/python/pyspark/__init__.py
index e39e6514d7..9556e4718e 100644
--- a/python/pyspark/__init__.py
+++ b/python/pyspark/__init__.py
@@ -37,16 +37,6 @@ Public classes:
 
 """
 
-# The following block allows us to import python's random instead of mllib.random for scripts in
-# mllib that depend on top level pyspark packages, which transitively depend on python's random.
-# Since Python's import logic looks for modules in the current package first, we eliminate
-# mllib.random as a candidate for C{import random} by removing the first search path, the script's
-# location, in order to force the loader to look in Python's top-level modules for C{random}.
-import sys
-s = sys.path.pop(0)
-import random
-sys.path.insert(0, s)
-
 from pyspark.conf import SparkConf
 from pyspark.context import SparkContext
 from pyspark.rdd import RDD