Speed up cloud-load test execution by retaining resources for quick consecutive runs

Introduced a feature to retain load agent resources for a configurable duration after a load test run finishes. This allows for quicker consecutive test iterations by reusing provisioned Azure resources, reducing the time spent on resource provisioning which can take up to 20 minutes. The feature is accessible via the 'Resources Retention' setting in Visual Studio 2015 Update 3 active run settings, or by adding a 'ResourcesRetentionTimeInMinutes' context parameter in older versions. Resources can be retained for a maximum of 240 minutes. A small surcharge of 5 VUMs per core per hour is applied for this feature. The feature is currently only available for load tests triggered using Visual Studio and will be brought to the web portal for other test types like Apache JMeter soon.