Allow configuration of update interval for ness_alarm

[nickw444]

Description:

Allow users to configure the polling update interval of the component. ref: https://community.home-assistant.io/t/ness-dx8-dx16-alarm/23285/90?u=nickw444

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#8720

Example entry for configuration.yaml (if applicable):

ness_alarm:
  host: alarm.home
  port: 8234
  update_interval:
    seconds: 10
  zones:
    - name: Garage
      id: 1
    - name: Storeroom
      id: 2

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.

[rohankapoorcom]

Thanks for using scan_interval instead of update_interval.