Add a metadata provider for Vultr

Vultr uses a very similar approach to AWS, including using the
same IP address for serving metadata. In fact, it seems
as though if AWS appears first in the list of providers, that
provider mistakenly believes to be running on AWS (hence the
insertion of `NewVultr` in between GCP and AWS. I don't believe
AWS servers will accidentally try to use the Vultr provider,
as it seems that the `/v1/` endpoint doesn't exist on AWS.

Signed-off-by: Luke Hodkinson <furious.luke@gmail.com>

pkg/metadata/main.go

@@ -47,7 +47,7 @@ var netProviders []Provider
 var cdromProviders []Provider
 
 func init() {
-	netProviders = []Provider{NewGCP(), NewAWS()}
+	netProviders = []Provider{NewGCP(), NewVultr(), NewAWS()}
 	cdromProviders = []Provider{NewCDROM()}
 }