[feature] Allow custom install/upgrade path in auto-install.sh

[c-gabri]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [x] I have updated the documentation.

Reference to Existing Issue

Closes #514.

Description of Changes

Let users specify a custom install/upgrade path when running auto-install.sh by passing it as an argument of --install/--upgrade. Document this feature.

Screenshot