EC2 plugin appears to assume that all nodes matching an AMI also have the label associated with that AMI. If that assumption is broken, it will select nodes that can't actually run the builds in the queue, and block launching new nodes.

To reproduce:

1. Add a new AMI with label "linux_test", and bring up at least one node with it.
2. In the AMI settings, change the label to "linux". Leave the existing node labeled "linux_test".
3. Run a build that's restricted to the "linux" label.
4. EC2 plugin enumerates all nodes matching the "linux" AMI, and selects the existing node.
5. Since the node only has the "linux_test" label, it can't run the build. Step 4 repeats indefinitely.

I used a workflow like the above to run verification builds on a new AMI using a test label, before assigning it the label associated with our main builds. Nodes used for the verification build must be deleted or have their label explicitly changed, or they block launching any additional nodes with the new AMI.