DefaultCreateCommand supports command w/ flags

namespaces.DefaultCreateCommand prepends the user-supplied command to
execute with "--", so that "nsinit init" does not attempt to interpret
it.

Before:

```
$ sudo nsinit exec -- ls -d
Incorrect Usage.

NAME:
   init - runs the init process inside the namespace

USAGE:
   command init [arguments...]

DESCRIPTION:


2014/07/31 16:57:55 flag provided but not defined: -d
2014/07/31 16:57:55 failed to exec: read parent syncpipe: connection reset by peer
```

After:

```
$ sudo nsinit exec -- ls -d
.
```