Make run_every and buffer_time to be consistent

[raymanound]

The run_every and buffer_time can be configured in the global config and each rule individually. I assume the rule settings will always take precedence to override the global config setting. However, they take precedence differently in different places in elastalert.py. For example,

elastalert2/elastalert/elastalert.py

Line 739 in f55421b

rule['starttime'] = rule.get('previous_endtime', endtime - self.run_every)

elastalert2/elastalert/elastalert.py

Line 748 in f55421b

rule['starttime'] = rule['starttime'] - (rule['buffer_time'] - rule['run_every'])

elastalert2/elastalert/elastalert.py

Line 599 in f55421b

buffer_time = rule.get('buffer_time', self.buffer_time)

I think we can set the rule['run_every'] and rule['buffer_time'] to have the default value from the global config at the first place and make the rest of the codes always reference rule instead of self to make it consistent. This can also benefit other modules that have a rule context, like Enhancements. What do you think?

[raymanound]

Feel like it is an issue more #1630