Support building an allow-list of providers (#527)

* Add providers subcommand to verify providers

Allows users to check what providers are available

* Fix version output to add new line

This command was totally broken until we moved around the call to
`initConfig()`, this just fixes the output now that it works.

* Flip boolean of provider include tags

All providers are still included by default and fix tags using the old
format.

providers/register/provider_alibabacloud.go

@@ -1,4 +1,4 @@
-// +build !no_alibabacloud_provider
+// +build alibabacloud_provider
 
 package register
 

providers/register/provider_aws.go

@@ -1,4 +1,4 @@
-// +build !no_aws_provider
+// +build aws_provider
 
 package register
 

providers/register/provider_azure.go

@@ -1,4 +1,4 @@
-// +build !no_azure_provider
+// +build azure_provider
 
 package register
 

providers/register/provider_azurebatch.go

@@ -1,4 +1,4 @@
-// +build !no_azurebatch_provider
+// +build azurebatch_provider
 
 package register
 

providers/register/provider_cri.go

@@ -1,4 +1,4 @@
-// +build linux,!no_cri_provider
+// +build linux,cri_provider
 
 package register
 

providers/register/provider_huawei.go

@@ -1,4 +1,4 @@
-// +build !no_huawei_provider
+// +build huawei_provider
 
 package register
 

providers/register/provider_mock.go

@@ -1,4 +1,4 @@
-// +build !no_mock_provider
+// +build mock_provider
 
 package register
 

providers/register/provider_nomad.go

@@ -1,4 +1,4 @@
-// +build !no_nomad_provider
+// +build nomad_provider
 
 package register
 

providers/register/provider_sfmesh.go

@@ -1,4 +1,4 @@
-// +build !no_sfmesh_provider
+// +build sfmesh_provider
 
 package register
 

providers/register/provider_vic.go

@@ -1,4 +1,4 @@
-// +build linux,!no_vic_provider
+// +build linux,vic_provider
 
 package register
 

providers/register/provider_web.go

@@ -1,4 +1,4 @@
-// +build !no_web_provider
+// +build web_provider
 
 package register
 

providers/register/register.go

@@ -1,6 +1,8 @@
 package register
 
 import (
+	"sort"
+
 	"github.com/cpuguy83/strongerrors"
 	"github.com/pkg/errors"
 	"github.com/virtual-kubelet/virtual-kubelet/manager"
@@ -30,6 +32,22 @@ func GetProvider(name string, cfg InitConfig) (providers.Provider, error) {
 	return f(cfg)
 }
 
+// Exists checks if a provider is regstered
+func Exists(name string) bool {
+	_, ok := providerInits[name]
+	return ok
+}
+
+// List gets the list of all provider names
+func List() []string {
+	ls := make([]string, 0, len(providerInits))
+	for name := range providerInits {
+		ls = append(ls, name)
+	}
+	sort.Strings(ls)
+	return ls
+}
+
 func register(name string, f initFunc) {
 	providerInits[name] = f
 }