lxd/containers_post: createFromMigration push mode
lxd/containers_post: createFromMigration push mode

Adapt createFromMigration() to support push mode. In push mode,
createFromMigration() will now return a operationClassWebsocket. Metadata() will
sent a set of websockets + secrets to the client.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@canonical.com>

mjeanson/lxd,akshaykarle/lxd,georghopp/lxd,Skarlso/lxd,atxwebs/lxd,pcdummy/lxd,chrisglass/lxd,mjeanson/lxd,techtonik/lxd,erikmack/lxd,stgraber/lxd,Skarlso/lxd,lxc/lxd,ollie314/lxd,chrisglass/lxd,techtonik/lxd,erikmack/lxd,techtonik/lxd,atxwebs/lxd,akshaykarle/lxd,hallyn/lxd,jsavikko/lxd,stgraber/lxd,georghopp/lxd,lxc/lxd,ollie314/lxd,hallyn/lxd,mjeanson/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,erikmack/lxd,stgraber/lxd,ollie314/lxd,lxc/lxd,stgraber/lxd,Skarlso/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,jsavikko/lxd,chrisglass/lxd,lxc/lxd,akshaykarle/lxd,pcdummy/lxd,hallyn/lxd,jsavikko/lxd,Skarlso/lxd,pcdummy/lxd,stgraber/lxd,georghopp/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd