Search for appropriate Python interpreter to use

[lordmauve]

The variation in Python interpreter paths means that it is always going to be unreliable to assume it's at a fixed location, such as /usr/bin/python{2,3}, as in the current implementation. Indeed we already work around this for the Docker Python images.

Instead, the bootstrap script could identify and exec an appropriate Python interpreter. By default we could just try python - which is likely to exist on the majority of systems - and switch if this is not correct and we can identify a more likely candidate.

Making this more difficult, there are several desirable properties of the current implementation to preserve.

• It should be possible to bootstrap the remote agent without disk write access.
• The bootstrap script should, if possible, be able to switch interpreter without re-sending the full bootstrap.
• We don't want to slow down the agent by having to proxy the I/O back to the agent, etc.