Implementing JENKINS-22141 Add Support for Dedicated Tenancy Option:

 - Added checkbox for dedicated tenancy to Slave Template.
 - Added help entry for dedicated tenancy option to clarify its usage.
 - Added support for dedicated tenancy for OnDemand Instances only, as this doesn't pertain to Spot Instances.
 - Updated SlaveTemplateTest.testConfigRoundtrip to ensure useDedicatedTenancy is set as expected.