reflect extraHosts of the hub container to node containers

[toshiya]

Add feature to reflect the extraHost setting of the hub container to node containers. (Discussed in the issue #182)

• code changes are only for DockerContainerClient.java, not consider the case of using kubernetes
• logic to read extraHosts from the hub is based on the existing getZaleniumNetwork

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (issue-182@6ad8d07). Click here to learn what that means.
The diff coverage is 75%.

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[diemol]

Hi @toshiya,

Thanks for this PR! I am on vacation now, I'll review it properly next week.

[diemol]

Hi @toshiya,

I'll merge this to a branch so I can add some documentation and then I'll merge to master.

Thanks!