lxd/instance/post: Update instancePostClusteringMigrate to respect instance's project
lxd/instance/post: Update instancePostClusteringMigrate to respect instance's project

Fixes #8484

Also renames instance argument to inst.

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

stgraber/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,atxwebs/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd