migrate 1.16 agent config to 1.18 properly (DataDir, Jobs, LogDir)

[Dimiter Naydenov]

We need tests that multiple upgrades from 1.16 to 1.18 and above work, as mentioned in bug 1290684.

The reason for the failure is that when reading a 1.16 agent config some values were not set before writing the migrated 1.18 agent config: DataDir (now required; it's either /var/lib/juju or $JUJU_HOME/local for the local provider), LogDir (optional; should default to /var/log/juju, except for the local provider), Jobs (optional, but we need to add them appropriately: JobHostUnits for all machines, except the bootstrap node; and JobManageEnviron+JobHostUnits for the bootstrap nodes, except for the local provider we need to skip JobHostUnits).