Add ability to easily append a flag to the default flags

[seans3]

As it stands now, it is not possible to append a flag to the default flags. Either you send all of your own flags, or you use only the default flags. For users who need to modify flags, this is a poor interface.

[mariantalla]

/assign @hoegaarden @mariantalla

[k8s-ci-robot]

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In response to this:

/assign @hoegaarden @mariantalla

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