enable vault integration

[pserranoa]

Implement and test vault integration. Enable the feature to present a VALID vault token to deploy an hcl with policy.

https://www.nomadproject.io/docs/configuration/vault.html https://www.nomadproject.io/docs/configuration/vault.html#allow_unauthenticated

Travis will test with all vault versions.

[pserranoa]

I have added a job that use a secret stored at vault, and use a container that present the secret as a http service to validate. I have implemented a test to check that the container has accesed correctly to the secret.

[codecov-io]

Codecov Report

Merging #78 into master will decrease coverage by 4.77%. The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   91.14%   86.37%   -4.78%     
==========================================
  Files          26       26              
  Lines        1028     1042      +14     
==========================================
- Hits          937      900      -37     
- Misses         91      142      +51

Impacted Files	Coverage Δ
nomad/__init__.py	94.68% <75%> (-3.13%)	:arrow_down:
nomad/api/base.py	89.65% <75%> (-7.88%)	:arrow_down:
nomad/api/acl.py	58.33% <0%> (-33.34%)	:arrow_down:
nomad/api/node.py	56.92% <0%> (-27.7%)	:arrow_down:
nomad/api/client.py	87.5% <0%> (-9.1%)	:arrow_down:
nomad/api/exceptions.py	92.3% <0%> (-7.7%)	:arrow_down:
nomad/api/metrics.py	92.3% <0%> (-7.7%)	:arrow_down:
nomad/api/jobs.py	87.23% <0%> (-2.13%)	:arrow_down:
... and 1 more

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[pserranoa]

I