Add quota attribute to volume section

[schaefi]

Allow to set quota per volume for the btrfs filesystem This Fixes #2651

[schaefi]

The integration test for TW worked

[   70s] [ DEBUG   ]: 12:03:50 | EXEC: [btrfs subvolume create /var/tmp/kiwi_volumes.29npkpkf/@/home]
[   70s] [ DEBUG   ]: 12:03:50 | Looking for btrfs in /sbin:/usr/sbin:/usr/local/sbin:/root/bin:/usr/local/bin:/usr/bin:/bin
[   70s] [ DEBUG   ]: 12:03:50 | EXEC: [btrfs quota enable --simple /var/tmp/kiwi_volumes.29npkpkf/@/home]
[   70s] [ DEBUG   ]: 12:03:50 | Looking for btrfs in /sbin:/usr/sbin:/usr/local/sbin:/root/bin:/usr/local/bin:/usr/bin:/bin
[   70s] [ DEBUG   ]: 12:03:50 | EXEC: [btrfs qgroup limit 5G /var/tmp/kiwi_volumes.29npkpkf/@/home]