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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