Main changes

• Firecracker support
• Add Sphinx docs
• Update supported OS, remove 5.8 ONE support

Other improvements or fixes

• Use iproute2 instead brctl
• Wait until the image is ready
• Support CentOS8 edge deployment
• Add DISK_TYPE to datastores
• Keep empty bridges
• Uninstall opennebula-rubygems when purging
• Let packages script install lxd snap
• Add openssh-server package to the list of common requirements.
• Fix get_interface_name()
• Run onprovision with debug when verbose
• correct APPS_URL
• Restart OpenNebula before running oneprovision
• Add --version to help
• Don't fail if hooks exist
• Parse Debian version correctly
• Other minor fixes