Investigate default VM lifetime in DockerDriver

[Jyrsa]

Currently blueprints on DockerDriver have lifetimes of 5h or 10h on Notebooks. Default lifetime for a host is 14400 seconds i.e. 4 hours. This creates interesting side effects when maxhosts is reached and no new hosts can be spawned but all hosts are already too old and won't be assigned new containers.

[tourunen]

I believe the host selection will use the hosts that are active but do not have lifetime left as last resort, so the above should not happen:

pebbles/pebbles/drivers/provisioning/docker_driver.py

Line 835 in 1f8c885

if len(selected_hosts) == 0:

Anyway, 4 hours is short if the instance lifetime is generally is longer than that.