Set the default path for an ISO library to `/opt/iso-images/` and add a new config flag to signal `hoster` where to look for ISOs by default

[yaroslav-gwit]

To simplify the ISO library management, I'll create a new dataset: /opt/iso-images/. It will then allow you to simply specify ISO name during the VM deployment like so: --from-iso file_name.iso, instead of the full path: --from-iso /opt/iso-images/file_name.iso.

An absolute path will still be supported, but vm deploy --from-iso will first check if the file exists in the /opt/iso-images/.

It will also allow you to use /opt/iso-images/ as a mount point, in case you'd like to share your library across multiple machines.

The improvement will also allow us to implement an easy way to deal with ISOs in the REST API and the WebUI.