lxc-to-lxd: Respect LXD_SOCKET environment variable
lxc-to-lxd: Respect LXD_SOCKET environment variable

The unix socket path of the LXD service can be customized via
LXD_SOCKET variable. Make sure this is also supported when migrating
containers from LXC to LXD.

Signed-off-by: Reto Gantenbein <192e2b4def8f7964936fd2f4aadf1f9332bed3b1@linuxmonk.ch>

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