Ensure curl is installed on hosts during bootstrapping

[simonrand]

Debian minimal (and others) doesn't come with curl installed out of the box, MRSK should check and resolve this during bootstrapping.

This is the root cause of the issue - but possibly worth opening an issue for the lack of information provided during failing healthchecks due to things like curl missing – i.e. no obvious error is raised beyond INFO Health check against /up failed to respond, retrying in Ns... – happy to open an issue for this if people would like.