Inventory: `SledRole::Gimlet` is misleading w.r.t. Cosmo

[jgallagher]

Our inventory system has this enum:

omicron/nexus-sled-agent-shared/src/inventory.rs

Lines 970 to 976 in fef36e5

	pub enum SledRole {
	/// The sled is a general compute sled.
	Gimlet,
	/// The sled is attached to the network switch, and has additional
	/// responsibilities.
	Scrimlet,
	}

The doc comments are correct, but the variant names are misleading; in particular, a non-scrimlet Cosmo sled in inventory will report it has a role of Gimlet.