Allow loading of a locustfile with multiple dots in filename

[raiyankamal]

Fix for #940
Uses methods from the importlib (python 3) and imp (python 2.7) modules to allow loading of files with multiple dots in the filename.

[codecov]

Codecov Report

Merging #941 into master will increase coverage by 0.83%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #941      +/-   ##
==========================================
+ Coverage   66.55%   67.38%   +0.83%     
==========================================
  Files          14       14              
  Lines        1438     1447       +9     
  Branches      226      226              
==========================================
+ Hits          957      975      +18     
+ Misses        430      418      -12     
- Partials       51       54       +3

Impacted Files	Coverage Δ
locust/main.py	33.18% <100%> (+7.01%)	⬆️

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[raiyankamal]

Closing because incomplete.

[raiyankamal]

Reopening PR after adding unit tests and correcting code for loading locustfile.

[cgoldberg]

thanks for contributing!