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

[c-gabri]

Is your feature request related to a problem? Please describe. auto-install.sh forces installation to /opt/openwisp, which may be undesirable. It also assumes /opt/openwisp as the installation path when upgrading.

Describe the solution you'd like Users should be able to specify a custom install/upgrade path by passing it as an argument to auto-install.sh and this feature should be documented.

Describe alternatives you've considered The Docker Compose installation method may be used to install OpenWISP anywhere, but it is less user-friendly and has no documented way to upgrade.

Additional context I'll create a PR related to this shortly.