Add datacenters data resource

[smaeda-ks]

maybe related: #523

$ FASTLY_API_KEY=**** make testacc TESTARGS='-run=TestAccFastlyDataSourceDatacenters'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccFastlyDataSourceDatacenters -parallel=4 -timeout 360m -ldflags="-X=github.com/fastly/terraform-provider-fastly/version.ProviderVersion=acc"
?   	github.com/fastly/terraform-provider-fastly	[no test files]
=== RUN   TestAccFastlyDataSourceDatacenters
=== PAUSE TestAccFastlyDataSourceDatacenters
=== CONT  TestAccFastlyDataSourceDatacenters
--- PASS: TestAccFastlyDataSourceDatacenters (1.34s)
PASS
ok  	github.com/fastly/terraform-provider-fastly/fastly	2.193s
?   	github.com/fastly/terraform-provider-fastly/fastly/hashcode	[no test files]
?   	github.com/fastly/terraform-provider-fastly/version	[no test files]

[Integralist]

Thanks @smaeda-ks for opening this PR. I had some questions, but otherwise this LGTM.

[Integralist]

The pattern across the code base so far has been to use fmt.Sprintf("%d", hashcode.String(...)) although that feels a bit ...hacky (for reasons I can't articulate) compared to using strconv.Itoa.

Seems others have written about the performance implications of using fmt.Sprintf (https://medium.com/swlh/bad-go-frivolous-sprintf-2ad28fedf1a0) and so it's probably worth at some point us updating the code to use strconv.Itoa.