Don't provision stack and keys if stack.json exists

[Maciej Kwidzinski]

It would allow for a fast rerun when:

• your temporary keys expired
• you want only to refresh uploads

Implementation consequences:

• still poll for stack status
• delete stack.json in cleanup-aws

[Maciej Kwidzinski]

It synergises with ticket #3

[Maciej Kwidzinski]

We should consider updating the stack instead of doing nothing.

[Maciej Kwidzinski]

This is different from ticket #1, because #1 should work despite the clean phase and this one is reset in the clean phase.

[Maciej Kwidzinski]

It might be pointless if provisioning is fast when ticket #1 is done