Fix an issue in types.ReconcileKey.String()

[briantkennedy]

While trying to determine what key to emit for an element without a
namespace, I looked at types.ReconcileKey which would emit "/name" which
doesn't appear to agree with the rest of the logic in kubebuilder. This
fixes types.ReconcileKey to emit the correct string value if namespace
is empty as well as adds a method to construct the object from a string.

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[pwittrock]

@briantkennedy Would you sign the cla bot?