improve spreadByField consts naming

[kevin-wangzefeng]

Signed-off-by: Kevin Wang kevinwzf0126@gmail.com

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind api-change

/kind bug

/kind cleanup

/kind deprecation
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
Improve the naming:

SpreadByCluster    =>    SpreadByFieldCluster 
SpreadByRegion     =>    SpreadByFieldRegion  
SpreadByZone       =>    SpreadByFieldZone    
SpreadByProvider   =>    SpreadByFieldProvider

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?: