Add the ability to specify when ILM step execution should occur

[dakrone]

When a user has a complex ILM policy for a production cluster with significant
load, some of the actions that ILM can take may be undesirable during peak
traffic hours (for example, an ecommerce site increasing replicas or performing
shard relocation for all indices at 8am on Black Friday morning).

A tool like Curator that is manually run is slightly more careful since it
allows the administrator to specify when the actions should take place (at 1am
when everyone is sleeping, for instance).

It would be nice if ILM could be configured to run its actions only during a
certain time frame, so actions could be kicked off at a "better" time.

There are a couple of options that I can think of quickly for how this could
be implemented:

• Add the ability to specify when ILM actions should be periodically checked,
  for instance, at 1am
• Specify a time range with a policy, for instance, 01:00-04:00 meaning that
  ILM will execute steps only between 1am and 4am (time on the master node)

[elasticmachine]

Pinging @elastic/es-core-features

[colings86]

Note that ILM can only control when a step is started so for things like forcemerge or allocation which may take a long time to complete ILM can only control when these changes are started and does not have any ability to pause the action mid-flow if the window closes

This is not a reason to avoid this feature but a implication we should be clear about in the docs

[dakrone]

True, the original issue above should be amended for the steps to say "start of execution" rather than step execution in general.

[steevo590]

I have the same concern would love to be able to specify when to start the step actions instead of just the size, documents or elapsed time.