reset the environment when the tunable parameters change

[motus]

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We still don't have it implemented. An environment must check when there is a change in the corresponding covariant group (simply by calling TunableGroup.is_updated("group_name")) and reset itself, if applicable. For example, if the vmSize parameter assumes a new value, VMEnv must re-provision a VM with the new size, and so on. The exact details should be specific to each Environment class implementation. We should also handle this situation correctly for composite/stacked environments (when the preceding environment resets the one higher on stack). For example, after re-provisioning a new VM we don't have to reboot it to reset the boot-time parameters. Most likely, we can address that at the tunables' level: i.e., reset the .is_updated flag of a tunable group if it is connected to some other group, etc.