Support "review_url" in .gitmodule

We have projects to build outside qt-project "gerrit instance", e.g. in github.
The qt-project "gerrit instance" is kind of hard coded in many places and defining a project outside it requires now full url:

.gitmodules:

 
[submodule "foobar"]
url = https://github.com/foo/foobar

Where as it could be:

[submodule "foobar"]
review_url: github
url = foo/foobar

i.e. the same way we define the projects residing in qt-project.

The "review_url" should be optional, default value being "qt-project".