Invalid backup files created by sed

[vholer]

Use of ${SED_I} in

https://github.com/OpenNebula/addon-context-linux/blob/master/src/etc/one-context.d/net-15-hostname#L5

causes the whole -i argument with parameter to be quoted. Backup file named with suffix '' is being created. Shouldn't break anything, only creates strange files. Preferably find a workaround to avoid using suspicious backup files, or fallback to a generic .bak suffix.