[SPARK-5366][EC2] Check the mode of private key

Check the mode of private key file.

Author: liuchang0812 <liuchang0812@gmail.com>

Closes #4162 from Liuchang0812/ec2-script and squashes the following commits:

fc37355 [liuchang0812] quota file name
01ed464 [liuchang0812] more output
ce2a207 [liuchang0812] pep8
f44efd2 [liuchang0812] move code to real_main
8475a54 [liuchang0812] fix bug
cd61a1a [liuchang0812] import stat
c106cb2 [liuchang0812] fix trivis bug
89c9953 [liuchang0812] more output about checking private key
1177a90 [liuchang0812] remove commet
41188ab [liuchang0812] check the mode of private key