Register exec driver plugin among some fixes

Namely, remove the `enabled` configuration flag, as it's specific to
`raw_exec` driver.  Also, pass resource limits to underlying call.

plugins/shared/catalog/register.go

@@ -1,6 +1,7 @@
 package catalog
 
 import (
+	"github.com/hashicorp/nomad/drivers/exec"
 	"github.com/hashicorp/nomad/drivers/qemu"
 	"github.com/hashicorp/nomad/drivers/rawexec"
 )
@@ -10,5 +11,6 @@ import (
 // register_XXX.go file.
 func init() {
 	RegisterDeferredConfig(rawexec.PluginID, rawexec.PluginConfig, rawexec.PluginLoader)
+	Register(exec.PluginID, exec.PluginConfig)
 	Register(qemu.PluginID, qemu.PluginConfig)
 }