Automate testing of provisioning changes

Currently, we are suffering from very slow process while making provisioning changes that need to added to qt/qt5 repository. To properly test a provisioning change, we will need to

1. Create a commit in qt5.git with the required provisioningchanges
2. Schedule a build with the previous commit ref
3. Wait until the machine is provisioned, which might take hours
4. Rebuild and test the module that is effecting the change

Steps 1-3 could essentially be replaced with a text file that would be executed prior to step 4. If this is a local file, the machine would be "provisioned" within minutes instead of hours. There is a way to do this also manually, but this is not trivial and the developers find the process off-putting and time-consuming. We would like to have a structural workflow to make this more approachable. 

Option 1) Agent can download custom provisioning script that is executed right before the build instructions

The webui would have functionality that would allow uploading custom provisioning script(s). 

Pros: Text files are small so there small data overhead and creating a vm is very fast.

Cons: More development effort and the provisioning change needs to be committed to the qt/qt5.git repository afterwards.

Option 2) Use tier3 templates that use the existing tier2 images as the base template

Instead of cloning the base template, we could using the pre-provisioned tier2 template as the base template and just add custom provisioning scripts on top of it.

Pros: Less changes required in code.

Cons: Template files are very large compared to the provisioning changes.