DOCloudInstance#containsAgent is not reliable

[beolnix]

DOCloudInstance#containsAgent doesn't look to be reliable. The best way is to match some agent property and started instance.

depends on:
#6
#7

See:
on the agent side:
https://github.com/JetBrains/teamcity-vmware-plugin/blob/master/cloud-vmware-agent/src/main/java/jetbrains/buildServer/clouds/vmware/VMWarePropertiesReader.java

on the server side:
https://github.com/JetBrains/teamcity-vmware-plugin/blob/master/cloud-vmware-server/src/main/java/jetbrains/buildServer/clouds/vmware/VmwareCloudInstance.java,
https://github.com/JetBrains/teamcity-vmware-plugin/blob/master/cloud-vmware-server/src/main/java/jetbrains/buildServer/clouds/vmware/connector/VMWareApiConnectorImpl.java (reconfigureInstance)