Tests - Simplify the various system property variables in pom.xml

[gbhat618]

Currently there is like 4 properties configured for tests purposes

google-compute-engine-plugin/pom.xml

Lines 352 to 355 in 8aeebbd

	<hudson.model.LoadStatistics.clock>500</hudson.model.LoadStatistics.clock>
	<hudson.model.LoadStatistics.decay>0.5</hudson.model.LoadStatistics.decay>
	<hudson.agents.NodeProvisioner.MARGIN>100</hudson.agents.NodeProvisioner.MARGIN>
	<hudson.agents.NodeProvisioner.MARGIN0>1.0</hudson.agents.NodeProvisioner.MARGIN0>

need to simplify as similar in kubernetes or mock-slave plugin.

See #495 (comment)

[jglick]

This is not just about tests—if you need to set these properties in order to get tests to run promptly, it is a sign that production code is lacking an optimized provisioning strategy.