feat(dask): Dask integration (#701)

[Alputer]

This pull request contains the ongoing work of dask integration into REANA.

[codecov]

Codecov Report

Attention: Patch coverage is 65.78947% with 13 lines in your changes missing coverage. Please review.

Project coverage is 60.06%. Comparing base (50495fc) to head (a7ea4f9).

Files with missing lines	Patch %	Lines
reana_server/validation.py	46.66%	8 Missing ⚠️
reana_server/rest/info.py	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #701      +/-   ##
==========================================
+ Coverage   60.01%   60.06%   +0.05%     
==========================================
  Files          32       32              
  Lines        3509     3546      +37     
==========================================
+ Hits         2106     2130      +24     
- Misses       1403     1416      +13     

Files with missing lines	Coverage Δ
reana_server/config.py	83.97% <100.00%> (+0.93%)	⬆️
reana_server/rest/workflows.py	51.13% <100.00%> (+0.09%)	⬆️
reana_server/rest/info.py	77.08% <58.33%> (-6.25%)	⬇️
reana_server/validation.py	70.73% <46.66%> (-5.74%)	⬇️