command init: show log output for each provider plugin downloaded

Each provider plugin will take at least a few seconds to download, so
providing feedback about each one should make users feel less like
Terraform has hung.

Ideally we'd show ongoing progress during the download, but that's not
possible without re-working go-getter, so we'll accept this as an interim
solution for now.

command/init.go

@@ -228,6 +228,7 @@ func (c *InitCommand) getProviders(path string, state *terraform.State) error {
 
 	dst := c.pluginDir()
 	for provider, reqd := range missing {
+		c.Ui.Output(fmt.Sprintf("- downloading plugin for provider %q...", provider))
 		err := c.getProvider(dst, provider, reqd.Versions, plugin.Handshake.ProtocolVersion)
 		// TODO: return all errors
 		if err != nil {