Remove providers (#670)

* Move all but mock provider out of tree

These have all been moved to repos under github.com/virtual-kubelet.

* Introduce a providers.Store

This essentially moves the the old register/ handling into a first class
object that can be controlled from the CLI rather than through build
tags deep in the code.

This actually would have made it a bit easier to build the provider
repos and makes the cmd/ code more re-usable.

providers/store.go

@@ -0,0 +1,73 @@
+package providers
+
+import (
+	"sync"
+
+	"github.com/virtual-kubelet/virtual-kubelet/errdefs"
+	"github.com/virtual-kubelet/virtual-kubelet/manager"
+)
+
+// Store is used for registering/fetching providers
+type Store struct {
+	mu sync.Mutex
+	ls map[string]InitFunc
+}
+
+func NewStore() *Store {
+	return &Store{
+		ls: make(map[string]InitFunc),
+	}
+}
+
+// Register registers a providers init func by name
+func (s *Store) Register(name string, f InitFunc) error {
+	if f == nil {
+		return errdefs.InvalidInput("provided init function cannot not be nil")
+	}
+	s.mu.Lock()
+	s.ls[name] = f
+	s.mu.Unlock()
+	return nil
+}
+
+// Get gets the registered init func for the given name
+// The returned function may be nil if the given name is not registered.
+func (s *Store) Get(name string) InitFunc {
+	s.mu.Lock()
+	f := s.ls[name]
+	s.mu.Unlock()
+	return f
+}
+
+// List lists all the registered providers
+func (s *Store) List() []string {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	ls := make([]string, 0, len(s.ls))
+	for p := range s.ls {
+		ls = append(ls, p)
+	}
+
+	return ls
+}
+
+// Exists returns if there is an init function registered under the provided name
+func (s *Store) Exists(name string) bool {
+	s.mu.Lock()
+	_, ok := s.ls[name]
+	s.mu.Unlock()
+	return ok
+}
+
+// InitConfig is the config passed to initialize a registered provider.
+type InitConfig struct {
+	ConfigPath      string
+	NodeName        string
+	OperatingSystem string
+	InternalIP      string
+	DaemonPort      int32
+	ResourceManager *manager.ResourceManager
+}
+
+type InitFunc func(InitConfig) (Provider, error)