[#4215] Have rundir depend on vardir

This came up because if you ran puppetd with a specific vardir, then
when rundir got set to a hardcoded value its parent directory might not
exist and the whole thing would fail.

This change came about with the concept of run_mode, and this fix is
restoring the behaviour that was in 0.25.x

Reviewed-by: Jesse Wolfe
Signed-off-by: Matt Robinson <matt@puppetlabs.com>