fix: ensure enabled instance if possible and set farm as render target

[sebastianbrandhuber]

Changelog Description

This PR fixes two issues:

• the active state of mandatory instances is not being changed now
• farm is set to the instances render target explicitly

Additional review information

This PR is meant to solve both these two issues:
Fix #261
Fix #257

Testing notes:

1. Create a nuke write instance, set the render target to something else than "Farm rendering"
2. Set the creator setting for the workfile as mandatory
3. Select the nuke write node, hit Render On Farm

[BigRoy]

code changes look good to me. @jakubjezek001 can you please confirm the enforcing of farm render target makes sense. It'd lose the ability to use existing frames on the farm through this button on the node, but likely that wasn't the intended usage of it anyway?