Fix selenium tests:

[ankitjavalkar]

Fix selenium tests:
- Fix addition of answer to codemirror using selenium
- Add custom exception when selenium fails
- Add id to logout button on student interface
- Fix imports for Python3 support in load_test.py

[ankitjavalkar]

I still need to keep selenium in install_requires because of the following reason;

I get this error when selenium is not present in the install requires;

This is not a problem with the module load_test, that file is present. As you can see below I have not installed selenium in this case (this is the only difference);

After adding the selenium to the install requires, all the modules are found properly and everything passes successfully.

[ankitjavalkar]

The tests are not passing because the Travis is pulling from the master branch of FOSSEE/online_test. Here is a local branch that is passing https://travis-ci.org/ankitjavalkar/online_test/builds/206595857

[prabhuramachandran]

Need to call raise SeleniumTestError(msg) instead of not using the message.

[ankitjavalkar]

fixed