Disabled agents or image configurations should not contribute capabilities to select in the UI

I keep finding that I'm selecting capabilities by accident which are only configured on disabled elastic bamboo image configurations and/or disabled agents.

For example, the default windows image has "JDK 1.6.0_29" and even though I've disabled that configuration, when selecting a JDK in Maven 3.x builder for example, this value still shows up in that list.

Since I can't actually delete the stock images, I'm forced to go and remove all of the capabilities from them to avoid folks accidentally using a capability which is only available on a disabled configuration.