fix overwrite of cluster-level default images

[frouioui]

As reported in #710, when a keyspace had keyspace-level image definition in its spec, it would overwirte the cluster-level image definition for any subsequent keyspaces.

This PR fixes #710 by doing a deep copy of the cluster-level image before assigning them to the keyspace.

This PR should be backported to all supported release branches.

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To give more context, these two lines were conflicting:

	planetscalev2.DefaultVitessKeyspaceImages(&images, &vt.Spec.Images)

	// Apply user-defined overrides for images.
	planetscalev2.MergeVitessKeyspaceTemplateImages(&images, &keyspace.Images)

Where DefaultVitessKeyspaceImages was assigning the VitessKeyspaceImages.Mysqld pointer of the cluster-level images to the keyspace-level, and then in MergeVitessKeyspaceTemplateImages we were modifying the value behind the pointer by the keyspace-specific image.