lxc/image: Add mode flag to image copy
lxc/image: Add mode flag to image copy

This adds the mode flag to image copy. It accepts the same values as for
instance copy: pull (default), push and relay.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

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