provider/vsphere: restore vcenter_server as deprecated field

As promised in my comment in #3718, this preserves backwards
compatibility while warning users of the new proper name for the field.

builtin/providers/vsphere/provider.go

@@ -29,6 +29,13 @@ func Provider() terraform.ResourceProvider {
 				DefaultFunc: schema.EnvDefaultFunc("VSPHERE_SERVER", nil),
 				Description: "The vSphere Server name for vSphere API operations.",
 			},
+
+			"vcenter_server": &schema.Schema{
+				Type:        schema.TypeString,
+				Optional:    true,
+				DefaultFunc: schema.EnvDefaultFunc("VSPHERE_VCENTER", nil),
+				Deprecated:  "This field has been renamed to vsphere_server.",
+			},
 		},
 
 		ResourcesMap: map[string]*schema.Resource{
@@ -40,10 +47,16 @@ func Provider() terraform.ResourceProvider {
 }
 
 func providerConfigure(d *schema.ResourceData) (interface{}, error) {
+	server := d.Get("vsphere_server").(string)
+
+	if server == "" && d.Get("vcenter_server").(string) != nil {
+		server = d.Get("vcenter_server").(string)
+	}
+
 	config := Config{
 		User:          d.Get("user").(string),
 		Password:      d.Get("password").(string),
-		VSphereServer: d.Get("vsphere_server").(string),
+		VSphereServer: server,
 	}
 
 	return config.Client()