[Reviewer: Alex] Capture all of /var/log in diags dumps

Fixes https://github.com/Metaswitch/clearwater-infrastructure/issues/60, fixes https://github.com/Metaswitch/chronos/issues/41.

This change captures all of /var/log rather than cherry-picking bits. This means we capture Chronos logs, and won't risk missing some log files if we add components in future.

I've checked three possible concerns:
* Clearwater diags are by far the largest logs - on the dogfood Sprout node, /var/log is 1.2GB and 1.1GB is /var/log/sprout, which we were collecting anyway.
* Nothing weird happens due to us writing to /var/log/clearwater-diags-monitor.log at the same time we copy it away.
* This does solve the original problem and copy off Chronos logs.