Placement scriptlet not called when creating an instance using --target flag

[victoitor]

When creating an instance using the --target flag to a specific cluster member and a placement scriptlet is defined through instances.placement.scriptlet, it is not called to check if placement restrictions apply. This can lead to inconsistent configurations similar to #589 and #1207 if the consistency of a configuration is checked through the placement scriptlet and user keys instead of through incus defined keys.

It should be noted that the placement scriptlet is called if --target is used with a cluster group, instead of a cluster member.

Even though the user sets the target, the scriptlet should be called having the target machine as the unique node in candidate_members as defined in the instance placement scriptlet documentation.

This change can better guarantee project rules defined through the placement scriptlet if it is used.