[FEATURE] Alerting within curator action

[karthikr77]

It would be good to have curator alert with a eMail when it completes a action/(set of actions). It can be made further configurable on when to alert viz. Success/Failure/Both.

This would avoid sending logs to LS/ES further to alert (assuming curator fails on working against a ES using its config)

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[petiepooo]

This could go hand in hand with #1692. If a surge of events hits Elasticsearch such that the disks get close to filling and Curator clears the oldest once out, receiving an alert that Curator took action would be useful to know it's time to take action. Possible actions: 1) adjust ILM to delete earlier, 2) expand nodes to gain more disk space, or 3) do nothing and let Curator deal with the surge.
There are other ways, of course, like processing Curator's json logs into ES and checking for task completion there, but sometimes it's simplest to just fire off an email from the source...