lxd/networks: Updates networkPost validation
lxd/networks: Updates networkPost validation

 - Moves "in use" check up from drivers, to reduce duplicate in per-driver Rename() functions.
 - Relaxes new name checks to only check for existing managed networks of the same name, removes checks for conflicting interface names (this only needs to be applied to bridge type networks and will be pushed down into bridge driver).
 - General clean up of comments and error messages.

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

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