config: add `reboot_window` for datetime constrained reboots

[lucab]

Airlock needs to have additional knobs to configure a "maintenance window" for reboots. Those needs to added both for the default group, and for specific custom groups.

A reboot window works in conjunction (logical AND) with existing counting-semaphore logic.

Entries to be added to the lock section:

• default_reboot_window_days: a set of short-day-names (like date %a, i.e. [ "Mon", "Wed" ])
• default_reboot_window_start: start time, in 24h format (i.e. 23:45)
• default_reboot_window_duration: length, in minutes

Entries to be added for each lock.groups section (these override defaults, if set):

• reboot_window_days
• reboot_window_start
• reboot_window_duration

Note: this relies on the system clock of the environment where an airlock process is running.

[arithx]

Should the window_start also include a timezone indicator and/or another variable specifying it? It seems reasonable that I might want to do all reboots within a specific timezone independent window (e.x. between 00:00 & 06:00 UTC) on a fleet of nodes that might be in multiple time zones.

[lucab]

TZ topic gets tricky quickly (countries changing the TZ they are in, TZ being redefined through the year, daylight-saving being a different TZ label, daylight switching). And I think locksmith was pretty much unaware of all of that (but I'd need to double check).

I don't have a good solution here. Perhaps one of these two approaches could work:

1. always use UTC (this likely works better in containers without tzdata file and TZ env)
2. always use local time (this is easier to reason about as a human, but depends on correct environment config, and can trip-off people in distributed environments)

Sidenote, if the environment is properly configured for UTC, option 2 could be seen a more complex/rich way to achieve option 1.

[arithx]

Maybe default to UTC and have a flag for using local time?

[lucab]

For reference, this is the set of configuration knobs we added to Zincati: https://coreos.github.io/zincati/usage/updates-strategy/#periodic-strategy