[swarming] Allow bot dimensions' values to be str/unicode
[swarming] Allow bot dimensions' values to be str/unicode

I think this means bot dimensions values can be str, i.e. not a list: https://cs.chromium.org/chromium/infra/luci/appengine/swarming/server/task_to_run_test.py?l=440-441&rcl=f8ec264ae66ab13a0a7fa53505715dfd3c8444d0

Bug: 1057886
Change-Id: Iabb33b7f99e176f3da20cc9b24ea26485bec0787
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2134334
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py