Hash login password on server side with bcrypt

[shreyamalviya]

Fixes #969
Uses bcrypt to salt and hash the password on the server side before storing it.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by running Island locally. Network traffic shows plaintext password. server_config.json stores salted and hashed password. Correct password results in authentication. Incorrect is denied.

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

Merging #1139 (d86e8e8) into develop (7f06ec4) will decrease coverage by 0.07%.
The diff coverage is 18.60%.

❗ Current head d86e8e8 differs from pull request most recent head 7772ea6. Consider uploading reports for the commit 7772ea6 to get more accurate results

@@ Coverage Diff @@ ## develop #1139 +/- ## =========================================== - Coverage 28.71% 28.64% -0.08%  =========================================== Files 410 411 +1 Lines 12877 12892 +15 =========================================== - Hits 3698 3693 -5  - Misses 9179 9199 +20 

Impacted Files	Coverage Δ
monkey/monkey_island/cc/resources/auth/auth.py	0.00% <0.00%> (ø)
.../monkey_island/cc/resources/auth/password_utils.py	0.00% <0.00%> (ø)
...ey/monkey_island/cc/resources/auth/registration.py	0.00% <0.00%> (ø)
monkey/monkey_island/cc/environment/standard.py	88.88% <66.66%> (+1.38%)	⬆️
...monkey_island/cc/environment/environment_config.py	100.00% <100.00%> (ø)
monkey/monkey_island/cc/environment/user_creds.py	100.00% <100.00%> (+6.89%)	⬆️

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