Revert "Merge pull request #25 from zzzeek/master"

This reverts commit de72f67a04ab01a3b73b44973fc730eb857913e2, reversing
changes made to 082f3e517d6172c0b8da896bd68773f14c77e6e7.
Reimplement instance cap management.

* Count instances created by EC2Cloud, not all instances in the Amazon EC2
account, when checking against the cloud's global instance cap.

* Inform each EC2Slave which SlaveTemplate it was created from, so instance
caps on templates can be managed correctly.

* Take into account slave templates which provide executor counts higher than 1
to avoid launching unnecessary instances.

* Track 'pending launch' nodes so that the instance cap can be strictly enforced.

Note that this patch requires some Jenkins core API changes (specifically to the
PlannedNode class contained in NodeProvisioner).