Use constants

[fabaff]

Description:

Use constants.

Example entry for configuration.yaml (if applicable):

notify:
  - platform: xmpp
    name: jabber
    sender: !secret xmpp_sender
    password: !secret xmpp_password
    recipient: !secret xmpp_recipient

Checklist:

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

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @balloob, @pvizeli and @MartinHjelmare to be potential reviewers.