allow forcing the server host and ip to be used for selenium tests
allow forcing the server host and ip to be used for selenium tests

This is useful when running selenium specs in an environment where you need to
specify an SSH-forwarded tunnel to communicate over, for example.

test plan: set server_ip and server_port in selenium.yml and run the selenium
tests. The remote selenium session should use those when accessing the test
host.

Change-Id: Iec5963a8c1481c25fa79820802c9b2ce35172f4a
Reviewed-on: https://gerrit.instructure.com/7248
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@instructure.com>
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

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