lxd/container/exec: Don't require cmd to be returned from inst.Exec()
lxd/container/exec: Don't require cmd to be returned from inst.Exec()

This isn't needed when using lxd-agent.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,hallyn/lxd,hallyn/lxd,Skarlso/lxd,Skarlso/lxd,Skarlso/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,mjeanson/lxd,atxwebs/lxd,mjeanson/lxd,mjeanson/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,Skarlso/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd