Run lxc commands as non root by default

[lucasmoura]

In ubuntu-advantage-tools, we require some LXD commands to be run as non-root. This PR changes the default behavior of LXD to run exec commands as non-root.

This will impact some cloud-init tests, but we already have a PR to handle that

[TheRealFalcon]

Actually...lxd is the exception for us as all other clouds already execute non-root. This may actually simplify things for us 🙂

[TheRealFalcon]

Travis is failing as this will break cloud-init tests, but once canonical/cloud-init#664 lands it should be a one-line fix. Since we've pinned our pycloudlib version, I'm not concerned these failures.

[blackboxsw]

Once canonical/cloud-init#694 lands, this looks good to land

[OddBloke]

The conclusion we came to in canonical/cloud-init#664 was that pycloudlib should consistently execute commands as an unprivileged user by default. This aligns LXD with the other clouds in pycloudlib which use SSH.

[OddBloke]

Per canonical/cloud-init#694 (comment), running commands as UID 1000 is not the same as running commands as self.username. I believe this will need modification to look up the UID of self.username before running lxc exec for the first time.

[lucasmoura]

@OddBloke I have updated the code to get the user id before running any lxc exec commands.

The solution I have is to retry the id -u command until the user was created. With that solution, I think we can drop from LXD the call for _wait_for_execute, since the user_id property is kind of covering that already

[OddBloke]

Thanks!