Preparations for GCP support

[alexott]

Added IsGcp & IsAws functions, changed relevant places when we have AWS specific functionality, etc.

[nfx]

Same build failure reason as in #559

[codecov]

Codecov Report

Merging #558 (4dd1317) into master (245efa6) will decrease coverage by 0.03%.
The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
- Coverage   81.62%   81.58%   -0.04%     
==========================================
  Files          77       77              
  Lines        6639     6642       +3     
==========================================
  Hits         5419     5419              
- Misses        811      814       +3     
  Partials      409      409              

Impacted Files	Coverage Δ
compute/clusters.go	76.56% <12.50%> (-0.33%)	⬇️
common/client.go	82.67% <33.33%> (-1.33%)	⬇️
compute/common_instances.go	76.47% <100.00%> (ø)
exporter/util.go	59.45% <100.00%> (ø)