[output] Adding support for assign priority to incidents

[javuto]

to: @ryandeivert @jacknagz
cc: @airbnb/streamalert-maintainers
size: small

Background

New output to use PagerDuty Incidents was added here. Incidents will be created with a low priority by default, unless specified.

Changes

• Added support to specify a valid priority name in the context.
• Added testing code.

Testing

$ rm .coverage && ./tests/scripts/unit_tests.sh
...
TOTAL                                                  2573     73    97%
----------------------------------------------------------------------
Ran 470 tests in 8.373s

OK

$ ./tests/scripts/pylint.sh

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

$ python manage.py lambda test --processor rule all
...
StreamAlertCLI [INFO]: (59/59) Successful Tests
StreamAlertCLI [INFO]: (93/93) Alert Tests Passed
StreamAlertCLI [INFO]: Completed

[coveralls]

Coverage increased (+0.03%) to 96.078% when pulling e423172 on javier-streamalert-priorities into dc1cec8 on master.

[ryandeivert]

Suggestion: pass the rule_context to the _priority_verify function and have it do all of this, and instead of having _priority_verify return False have it return the default value of {} or dict()..

So your call would just look something like:
incident_priority = self._priority_verify(rule_context)

And everywhere above that returns false would instead return dict()

[coveralls]

Coverage increased (+0.03%) to 96.078% when pulling 2a454e5 on javier-streamalert-priorities into a5dd27e on master.

[ryandeivert]

Thanks for the change! Looks great - one last suggestion: don't instantiate the incident_priority here or do the if rule_context check here.. do this all in the _priority_verify and return dict() immediately if the rule_context == None

[ryandeivert]

Approving but please see tiny nit comment

[coveralls]

Coverage increased (+0.03%) to 96.078% when pulling 6099948 on javier-streamalert-priorities into a5dd27e on master.