Elastic instances don't get started because LocalAgentManagerImpl.getExecutableAgents returns offline agents

see screenshot and compare it with the log entries bellow:

2010-06-01 10:55:05,608 DEBUG [QuartzScheduler_Worker-8] [LocalAgentManagerImpl] Trying to match requirements... com.atlassian.bamboo.v2.build.agent.capability.RequirementSetImpl@5c8c9c5d[[system.jdk.JDK LIKE .*, system.builder.mvn2.Maven 2.1 LIKE .*]]
2010-06-01 10:55:05,609 DEBUG [QuartzScheduler_Worker-8] [LocalAgentManagerImpl] Added agent panda.sydney.atlassian.com(id = 84017158)

LocalAgentManagerImpl.getExecutableAgents should not return offline agents. Currently it returns them and it leads to a bug in "Automatic elastic instance management" code. This happens because when we check if any enabled agents can run a build we find that there is one and don't start a new one.

The problem might also cause issues in other places.