upgrade unattended-upgrades on deploy (#6245)

unattended-upgrades is not able to upgrade itself in certain situations,
such as when the conffile prompt is generated due to the config being
changed. We want to set this package as latest in the platform so that
it is upgraded on every deploy (we deploy the config anyway).

Change-Id: I8c99bfb1b001079f0e1a4ffbf048e0e867633335
Merge "upgrade unattended-upgrades on deploy (#6245)" into develop