Process definition for backup-steps

[wachterjohannes]

The backup command will have a new options --process files which then only executes the backup-steps which has the process defined. All the other backups will be ignored. The default is all steps.

The backup will have a new database entry with process which will be used in restore command.

{
    "name": "application",
    "backup": {
        "data": {
            "plugin": "directory",
            "process": ["files"],
            "parameter": {
            }
        },
       {
            "plugin": "mysql",
            "process": ["databases"],
            "parameter": {
            }
        }
    }
}

@chirimoya @c00n64 can you add some thoughts?