health-check: Adding write support

By using DESTRUCTIVE_MODE environement variable, the storage performance
mode will generate writes.

As per the read test, randwrite at 4K at seq write at 1MB loads are generated.

A typical output will be:

 ('disk', 'sdb', 'standalone_randread_4k_iops', '2408'),
 ('disk', 'sdb', 'standalone_read_1M_bwps', 162506),
 ('disk', 'sdb', 'standalone_read_1M_iops', '155'),
 ('disk', 'sdb', 'standalone_randwrite_4k_bwps', 43),
 ('disk', 'sdb', 'standalone_randwrite_4k_iops', '8'),
 ('disk', 'sdb', 'standalone_write_1M_bwps', 30425),
 ('disk', 'sdb', 'standalone_write_1M_iops', '26'),
 ('disk', 'sdb', 'standalone_randread_4k_bwps', 90),
 ('disk', 'sdb', 'standalone_randread_4k_iops', '19'),
 ('disk', 'sdb', 'standalone_read_1M_bwps', 76208),
 ('disk', 'sdb', 'standalone_read_1M_iops', '71'),
 ('disk', 'sdb', 'standalone_randwrite_4k_bwps', 79),
 ('disk', 'sdb', 'standalone_randwrite_4k_iops', '17'),
 ('disk', 'sdb', 'standalone_write_1M_bwps', 111199),
 ('disk', 'sdb', 'standalone_write_1M_iops', '105')
health-check: Reporting bandwidth unit

In fact, that's now KB per sec.
Removing a debug line that doesn't deserved to be here.