[scripts][pipelines] allow dask-clients as strings

Some strings like single-threaded, processes, ... are now allowed as
options for the dask-client click option. Depends on bob.extension!122
2 jobs for dask-client-as-string in 7 minutes and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #219108
docker
build_linux_37

00:07:38

83.0%
passed #219107
macosx
build_macosx_37

00:06:16

83.0%