[FIXED JENKINS-23850] Workaround AWS bug that hides stopped slaves

Use instance.group-name instead of group-name as the describeInstances
filter when searching for stopped slaves to restart.

Some EC2 regions are incorrectly returning empty describeInstances
results when the group-name filter is used. This causes jenkins to start
new slaves instead of restarting stopped instances.

instance.group-name is functionally identical and seems to work
consistently.

https://issues.jenkins-ci.org/browse/JENKINS-23850
Merge pull request #99 from acoulton/bug/23850-always-new-slaves-in-eu-west

Workaround AWS bug that hides stopped slaves