Make test data server configurable

[ytremolet]

The server http://download.ecmwf.org is hardcoded in several places in ecbuild_get_test_data.cmake
It would be better if the server name be made configurable so we can use a server closer to us, add test data, etc...

[tlmquintino]

This should now be addressed in the develop branch

[tlmquintino]

A fix has been implemented in 2815760

[tlmquintino]

Define ECBUILD_DOWNLOAD_BASE_URL as the base for URL construction.

[wdeconinck]

I have a question..
How do we manage a bundle where test-data may possibly be distributed over multiple test-data servers?
We would have to modify the variable "ECBUILD_DOWNLOAD_BASE_URL" for each subproject.

[tlmquintino]

I think ECBUILD_DOWNLOAD_BASE_URL should be a default and each project should set its own variable _DOWNLOAD_BASE_URL