- add a new mechanism to help count the total number of EC2 instances for a particular AMI.  As an EC2Slave
is being provisioned, a temporary count is placed in a HashMap, which is used in addition to the
count reported by Amazon itself for a particular ami.   This is so that the count of total
nodes provisioned takes into account those which Amazon doesn't report yet.  The count returned
may be too high, if amazon reports it in addition to the "provision" count, but better to err on the
side of not spawning a node too soon; it will get spawned on the next go-around.
Tries to fix [JENKINS-6691].
Merge pull request #25 from zzzeek/master

keep track of instances being provisioned; use this count when determining total/AMI instance caps