Resolve "enable more checks from golangci-lint"

Closes #114 (closed)

Changes include:

• automatic fixes by golangci-lint
• added config for golangci-lint in backend/.golangci.yml
• added arg fix to lint_go take task that enables fixing
• added ignoring cover check for new func prepareTLC
• manual fixes for golangci-lint
  • removed all globals
  • added CAClient to avoid httpClient11 global, this involves creating NewStorkAgent function, and replaces client.go
  • reorganized imports, now they are in the following order (separated by empty line): stdlib imports, 3rd pardy packages, our stork packages
  • all acronyms are using capital letters now (eg. URL instead of Url)
  • replaces app types from explicit strings like "kea" to consts like KeaAppType
  • machineToRestApi func does not return error anymore so error handling has been removed
  • split Serve function from restservice.go to reduce its complexity
  • reordered return values so error is always last