Fix alerta node hang by adding http timeout and fix time.Duration convert when define topic handler with httppost

[Ehekatl]

After run into kapacitor tasks hang several time, I found it's caused by lack of http timeout support in alerta service.

this pr also fixed one of many issues where you can't use timeout in string within topic handler options.
I only did it for http post though.

e.g.

---
id: test
topic: test
kind: post
match: level() == CRITICAL
options:
  endpoint: 'media-as2s'
  timeout: 10s

Required for all non-trivial PRs

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