add the ability to configure run instance count under jobs settings

[biharck]

What

Currently, GoCD allows jobs be played in parallel and under the jobs settings there is an option called runInstanceCount which basically defines how many instances will run the job in parallel.

The documentation can be found at https://docs.gocd.org/current/advanced_usage/admin_spawn_multiple_jobs.html

Why

Copied from the documentation:

If you want to run multiple instances of the same job configuration you do not have to maintain multiple copies of same job config. You can specify how many instances of job you need & Go will take care of spawing the required number of job instances during scheduling.

This feature is particularly useful for test parallelization. It enables Go users to integrate with other test parallelization tools like TLB etc. to achieve distributed test execution with minimal configuration.

This PR is just to allow this implementation from Gomatic.

[ghost]

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