best effort commercial server type validation

[nicolai86]

fetch the list of known server types on initialization and cache it to validate r/server types later.

closes #17

all server-related tests remain green with this change:

make testacc TESTARGS="-run='TestAccScalewayServer_'"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run='TestAccScalewayServer_' -timeout 120m
?   	github.com/terraform-providers/terraform-provider-scaleway	[no test files]
=== RUN   TestAccScalewayServer_importBasic
--- PASS: TestAccScalewayServer_importBasic (91.70s)
=== RUN   TestAccScalewayServer_Basic
--- PASS: TestAccScalewayServer_Basic (96.95s)
=== RUN   TestAccScalewayServer_ExistingIP
--- PASS: TestAccScalewayServer_ExistingIP (88.13s)
=== RUN   TestAccScalewayServer_Volumes
--- PASS: TestAccScalewayServer_Volumes (105.94s)
=== RUN   TestAccScalewayServer_SecurityGroup
--- PASS: TestAccScalewayServer_SecurityGroup (124.18s)
PASS
ok  	github.com/terraform-providers/terraform-provider-scaleway/scaleway	506.928s

[nicolai86]

Pulling this in optimistically.