[MNG-5615] Rework credentials handling

**[Anders Hammar](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=afloom)** opened **[MNG-5615](https://issues.apache.org/jira/browse/MNG-5615?redirect=false)** and commented

It would be great with a way to handle credentials outside of settings.xml. Preferably in a pluggable manner so that it can be integratedwith corporate environments for example.

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**Issue Links:**
- [MNG-5913](https://issues.apache.org/jira/browse/MNG-5913) Allow to define aliases for existing server configurations in settings.xml

- [MNG-5356](https://issues.apache.org/jira/browse/MNG-5356) Make encrypt/decrypt logic pluggable

- [MSITE-356](https://issues.apache.org/jira/browse/MSITE-356) Be able to provide username/password as CLI parameters (and not only from settings.xml)
(_**"supercedes"**_)