Add a datacenter view to access information about the Sensu servers

[palourde]

Description

We add a new datacenter view, accessible from the datacenters view, in order to access new information about the Sensu servers available since Sensu 0.28.

The Datacenter structure exposed in the /datacenters endpoint has been modified; the Stats attribute is now called Metrics.

We also add additional debug information when Sensu API does not respond properly.

Related Issue

Fixes #652
Depends on https://github.com/sensu/uchiwa-web/pull/145

Motivation and Context

Display Sensu servers information

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #660 into master will decrease coverage by 0.39%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #660     +/-   ##
=========================================
- Coverage   22.48%   22.08%   -0.4%     
=========================================
  Files          26       27      +1     
  Lines        2135     2173     +38     
=========================================
  Hits          480      480             
- Misses       1598     1636     +38     
  Partials       57       57

Impacted Files	Coverage Δ
uchiwa/daemon/daemon.go	13.22% <0%> (-0.82%)	⬇️
uchiwa/datacenter.go	0% <0%> (ø)
uchiwa/server.go	0% <0%> (ø)	⬆️
uchiwa/daemon/datacenters.go	0% <0%> (ø)	⬆️

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