DEPL4 — ArtifactDeploymentDescription: "node" should be optional

In section 6.8.2.2 (ArtifactDeploymentDescription attributes),
the text for the "node" attribute explains,

node: String
The name of the node where the artifact is to be installed.
If blank, the node is implied by the InstanceDeploynment-
Description parent.

The empty string should not have a special meaning. Rather,
the attribute should be optional.

Proposed resolution:

Change the attribute to have 0..1 multiplicity, and change
the text to read as follows:

node: String [0..1]
The name of the node where the artifact is to be installed.
If missing, the node is implied by the InstanceDeployment-
Description parent.