lxd/api/cluster: Adds mutex to clusterNodesPost to prevent concurrent requests creating duplicates
lxd/api/cluster: Adds mutex to clusterNodesPost to prevent concurrent requests creating duplicates

Fixes #9183

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

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