Draft: Add `enable-disk` flag

What does this MR do?

Introduces the flag enable-disk=true by default. This flag will allow people use NFS instead of Object Storage.

When enable-disk=false the NFS mount won't be needed anymore and Pages will not try to scan the directory for files to serve.

This flag will replace domain-config-source starting from %14.0, so this change feels redundant with domain-config-source and use-legacy-store together but it will be clearer in once we remove domain-config-source in #382 (closed)

Depends on !457 (merged)

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required)
• Code review guidelines
• Go Style guidelines

Related to #543 (closed)