change the way the selenium in-proc server chooses ports
change the way the selenium in-proc server chooses ports

the process of selecting ports at random is an error-prone and slow
way of selecting a free port. instead, we simply bind against port 0,
which indicates to the OS to select a free port for us.

test plan:
 - selenium tests pass

Change-Id: I86dd05aaa469f8f2765ec5d56fff5f1e812cbdfc
Reviewed-on: https://gerrit.instructure.com/24728
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Bryan Madsen <0a3ba1af216659500dd8f41e29f3979f42452870@instructure.com>
Product-Review: Bryan Madsen <0a3ba1af216659500dd8f41e29f3979f42452870@instructure.com>
QA-Review: Bryan Madsen <0a3ba1af216659500dd8f41e29f3979f42452870@instructure.com>

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