Make the host of project template flexible

What does this MR do and why?

In the JH addition, we already override the preview of every project template, like:

Project Template	Current Preview	JH Preview
rails	https://gitlab.com/gitlab-org/project-templates/rails	https://jihulab.com/gitlab-cn/project-templates/rails
spring	https://gitlab.com/gitlab-org/project-templates/spring	https://jihulab.com/gitlab-cn/project-templates/spring
express	https://gitlab.com/gitlab-org/project-templates/express	https://jihulab.com/gitlab-cn/project-templates/express

In the file lib/tasks/gitlab/update_templates.rake and the file spec/tasks/gitlab/update_templates_rake_spec.rb, there are some hardcode of the preview. Currently we skipped this test, but it would be better that enable this test again after they were changed flexibly:

# lib/tasks/gitlab/update_templates.rake
get_commits_response = Gitlab::HTTP.get("https://gitlab.com/api/v4/projects/#{uri_encoded_project_path}/repository/commits",

# spec/tasks/gitlab/update_templates_rake_spec.rb
stub_request(:get, %r{^https://.*/api/v4/projects/gitlab-org%2Fproject-templates%2Frails/repository/commits\?page=1&per_page=1})

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How to set up and validate locally

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

/cc EM @daveliu