Provide a mechanism to allow updates in certain time window

[gthao313]

users could have workload that cannot be interrupted during some periods of the day, so we provide a mechanism to prevent Bottlerocket nodes update on user customized time window.

Issue number:

Closes: #67

Description of changes:

Author: Tianhao Geng <[email protected]>

Provide a mechanism to allow updates in certain update time window

Users could have workload that cannot be interrupted during some periods
of the day, so we provide a mechanism to allow Bottlerocket nodes updates
on user customized update time window. Beyond update time window,
bottlerocker update operator will prevent any updates.

Testing done: Set up update time window and then confirm if brupop stops updating nodes except in-process node.

UPDATE_WINDOW_START=23:55:0
UPDATE_WINDOW_STOP=0:10:0

UPDATE_WINDOW_START=0:15:0
UPDATE_WINDOW_STOP=0:24:0

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[gthao313]

This PR has conflicts with #238. I'll solve the conflicts when #238 is merged.

[gthao313]

Push above:

1. Add a buffer 6 mins before time window coming - this can make sure that brupop completes in-process updates and doesn't have any actions when time window starts. 2.Add a strategy to deal with if window start time later than end time - will recognize it as cross day period. like 11pm to 2am.
2. rename time window related env variable.
3. rebase

[gthao313]

Push above changes logic from prevent update time window to update time window - "maintenance window".

[gthao313]

Push above Rebase and fix README according to Matt's comments.