feature: add retain-handle flag in disk fill experiment

[xcaspar]

Signed-off-by: xcaspar x.caspar@gmail.com

Describe what this PR does / why we need it

chaosblade-io/chaosblade#325

Does this pull request fix one issue?

Describe how you did it

Add retain-handle flag to hold the file handle.

Describe how to verify it

blade c disk fill --retain-handle --percent 50

Use lsof command to lookup file handle:

 lsof | grep chaos_filldisk.log.dat
chaos_fil 26833          root    3r      REG              253,1 6320816128         18 /chaos_filldisk.log.dat
chaos_fil 26833 26834    root    3r      REG              253,1 6320816128         18 /chaos_filldisk.log.dat
chaos_fil 26833 26835    root    3r      REG              253,1 6320816128         18 /chaos_filldisk.log.dat
chaos_fil 26833 26836    root    3r      REG              253,1 6320816128         18 /chaos_filldisk.log.dat
chaos_fil 26833 26838    root    3r      REG              253,1 6320816128         18 /chaos_filldisk.log.dat

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