[wmi] extend tagging for related class & constants

[yannmh]

Rebased #1253 on 5.2.0 agent release.

Added a few changes:

• String cast and flatten link_value tags values (https://github.com/DataDog/dd-agent/compare/yann/wmi-extend-tagging?expand=1#diff-7a46560288aed829df125f0734b355f9R89)
• Cast source property link value to int instead of float (experienced issues in my tests)

Thanks a lot @rlaveycal

[LeoCavaille]

typo Setting

[LeoCavaille]

Thanks Yann, looking good!

Just two nitpicky comments, to make it look even nicer:

• can you make the commit messages follow these guidelines
  • short title are great to see in a git log (< 72 chars)
  • flags, like [wmi] are easy to grep for
• can you keep the same spacing conventions in the YAML file it
  should be only 2 spaces comments

[yannmh]

Thanks @LeoCavaille !

[LeoCavaille]

Tried it on my box @yannmh , works well.

• One thing I am slightly worried about is the number of tags your example can create, on my simple box with nothing running on it: ~40 commandline and ~100 name tags.
  Would this be possible to comment all examples in the sample file and let the user uncomment what he really wants?
  Can we also remove the allapps example, it's not helpful?
• Second remark: looks like we cannot use several link_tag lists? Do you think it's a valid usecase?

[LeoCavaille]

Also, maybe reword the comment for link_tag

 # `link_tag` optionally specifies how to join to another property for tagging.
 # Setting this will cause any instance number to be removed from tag_by values
 # i.e. name:process#1 => name:process
 # `link_tag` is a list of four parameters: [param1, param2, param3, param4]
 #     param1: property of source that contains the link value
 #     param2: class to link to
 #     param3: property of target class to link to
 #     param4: property of target class that contains the value to tag with

[yannmh]

Thanks @LeoCavaille. I brought some changes following your feedback.

I am merging it when Travis tests are passed,

[coveralls]

Changes Unknown when pulling d1af6ea on yann/wmi-extend-tagging into * on master*.