feat(alerta-service): Add custom severities to Alerta handler

[sp1r]

Fix #2056

This commit allows usage of all or some Alerta Severities. It provides two keywords to fine tune kapacitor built-in severities.

1. First, you can rename kapacitor serverity levels: crit, warn, info, ok to any other severities configured in your Alerta:

   |alert()
   // ...
   .alerta()
   // ...
   .renameSeverity('crit', 'major')
   .renameSeverity('info', 'notice')

I suppose this will cover most of the cases. But if you do want a lot of severity levels:

2. You can add custom severity levels, which will be avaluated on Alerta handler level after built-in alert was triggered.

   |alert()
   // ...
   .warn(lambda: "cpu" > 50)
   .alerta()
   // ...
   .addSeverity('minor', 3, lambda: "cpu" > 60)
   .addSeverity('major', 2, lambda: "cpu" > 70)
   .addSeverity('critical', 1, lambda: "cpu" > 80)
   .addSeverity('fatal', 0, lambda: "cpu" > 90)

Note: evaluation of addSeverity condition only happen after build-in alert is triggered, so you need some entry point (like .warn() in exmple), which should cover all range of values interesting to you.
Note: this severities use Alerta's code order - higher severity has lower code (0 for fatal, 9 for ok)
Note: .addSeverity() is quite useless in combination with .stateChangesOnly(), but Alerta has decent deduplication mechanism, so it shouldn't be a problem

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated
• Sign CLA (if not already signed)