Enable to set the distribution of an empty file user set xattr
This feature is for test purpose. It enables to decide for each file before
it is written the distribution

[root@localhost mnt1]# attr -g rozofs z
Attribute "rozofs" had a 98 byte value for z:
EID 1 / LAYOUT 0 / FID c44fd924-fe0c-4e6c-8749-87fd12f566ba / CLUSTER 1 / STORAGED 001-002-003-004
[root@localhost mnt1]# attr -s rozofs -V "2 5-6-7-8" z
Attribute "rozofs" set to a 9 byte value for z:
2 5-6-7-8
[root@localhost mnt1]# attr -g rozofs z
Attribute "rozofs" had a 98 byte value for z:
EID 1 / LAYOUT 0 / FID c44fd924-fe0c-4e6c-8749-87fd12f566ba / CLUSTER 2 / STORAGED 005-006-007-008
Minor debug enhancements