environs: add NewEnviron(kind, config)

environs/open.go

 package environs
 
 import "fmt"
 
-// New creates a new Environ using the
-// environment configuration with the given name.
-// If name is empty, the default environment will be used.
+// Open creates a new Environ using the environment configuration with the 
+// given name. If name is empty, the default environment will be used.
 func (envs *Environs) Open(name string) (Environ, error) {
         if name == "" {
                 name = envs.Default
+                if name == "" {
+                        return nil, fmt.Errorf("no default environment found")
+                }
+        }
+        e, ok := envs.environs[name]
+        if !ok {
+                return nil, fmt.Errorf("unknown environment %q", name)
+        }
+        if e.err != nil {
+                return nil, e.err
+        }
+        env, err := providers[e.kind].Open(name, e.config)
+        if err != nil {
+                return nil, fmt.Errorf("cannot initialize environment %q: %v", name, err)
+        }
+
+        return env, nil
+}
+
+// NewEnviron creates a new Environ of the registered kind using the configuration
+// supplied.
+func NewEnviron(kind string, config map[string]interface{}) (Environ, error) {
+        p, ok := providers[kind]
+        if !ok {
+                return nil, fmt.Errorf("no registered provider for kind: %q", kind)
+        }
+        cfg, err := p.ConfigChecker().Coerce(config, nil)
+        if err != nil {
+                return nil, fmt.Errorf("error validating environment: %v", err)
+        }
+
+        // TODO(dfc) the name of the environment needs to be injected when it is······
+        // created from environments.yaml
+        return p.Open("default", cfg)
+}