Fix template max

[kmalino66]

This pull request adds a JenkinsTemplateTag tag to the ARM templates for creating the virtual machines in Azure with the value of the template name. Fixes #563 and Fixes #378.

With existing state, whenever AzureVMManagementServiceDelegate.getVirtualMachineCountsByTemplate() is run, it is looking for this tag to build a map to track the number of existing agents (lines 1843-1854). Since this tag does not exist, the resulting map does not correctly reflect the count of VMs which exist.

Testing done

First started creating a Jenkins server using the existing released version of the plugin. Confirmed we were able to reproduce the error, in this case by running multiple pipelines scoped to the agent label with a configured maximum template limit of 1. Was able to successfully reproduce the issue.

Built and installed plugin with existing code into same instance of Jenkins and recreated the agents. Performed same test as above, and only 1 agent was able to be created. I confirmed the presence of log messages similar to the following: Wanted to create 1 nodes from template jnksagtqa3 but cannot create any, have template limit of 1 but have 1 VMs already so we can have 0 more, currently have 1 VMs in cloud. Continued to run this to ensure we meet a scheduled run of the verification task, which would historically break the counts.

The templates used are configured with a custom image, managed disks, and startup script. This means customImageTemplateWithScriptAndManagedDisk.json is the main template tested so far.

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• Ensure that the pull request title represents the desired changelog entry
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[timja]

Thanks! (untested)