Keep value of metal_connection.vlans in sync with API

[ctreatma]

This updates the metal_connection resource and data source to read the value of vlans from the API response to ensure that terraform state stays up-to-date with the real infrastructure if someone modifies the VLANs for a terraform-managed metal_connection outside of terraform.

Closes #270

[codecov-commenter]

Codecov Report

Base: 59.43% // Head: 60.17% // Increases project coverage by +0.74% 🎉

Coverage data is based on head (687c986) compared to base (a9f11e3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
+ Coverage   59.43%   60.17%   +0.74%     
==========================================
  Files          74       74              
  Lines       12889    12911      +22     
==========================================
+ Hits         7660     7769     +109     
+ Misses       4937     4839      -98     
- Partials      292      303      +11     

Impacted Files	Coverage Δ
equinix/data_source_metal_connection.go	98.78% <100.00%> (+0.09%)	⬆️
equinix/resource_metal_connection.go	71.54% <100.00%> (+1.17%)	⬆️
equinix/resource_metal_spot_market_request.go	52.07% <0.00%> (-7.55%)	⬇️
equinix/data_source_metal_virtual_circuit.go	96.36% <0.00%> (-3.64%)	⬇️
equinix/helpers_device.go	67.52% <0.00%> (-0.52%)	⬇️
equinix/resource_metal_virtual_circuit.go	76.35% <0.00%> (-0.32%)	⬇️
equinix/resource_metal_port_vlan_attachment.go	65.50% <0.00%> (+3.49%)	⬆️
equinix/resource_metal_device.go	69.10% <0.00%> (+5.49%)	⬆️
equinix/utils.go	86.95% <0.00%> (+10.86%)	⬆️
... and 1 more

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