The script phase should have a retry option

[Laurent Sesquès]

In order to automate deployments, it would be nice for a script phase to be able to retry.
It can't be implemented in the script which is called, because mojo buffers the output until the command completes.
Here's an MP implementing it:
https://code.launchpad.net/~sajoupa/mojo/script-phase-retry/+merge/349817
I tested very basically. Script with retry works as expected, and verify with retry is still ok (it inherits from script and already had retry implemented).

[Tom Haddon (mthaddon)]

I'm not convinced we actually need this. I suspect most times this can be better expressed with the verify phase for things that need retrying. If there's a compelling reason for it, I could be convinced the other way though.