Add option that will load all the config files in a directory

[ekouts]

In cscs we have (almost) separated all the systems configuration in different files for better readability but now we have to pass them all manually in the cli -C common.py -C daint.py -C eiger.py -C .... It would be convenient if Reframe had an option that would look for all the configuration files in a directory.

[vkarak]

We don't need necessarily a new option; it could be the same option if it is passed a directory to load all the python files inside. The problem is the order as it matters for loading. What would that be?

[ekouts]

In our case it shouldn't matter, things should not be duplicated (unless they are targeting different systems). I would say alphabetical and we just document it. As long as we document it, if someone needs a specific order they could name them accordingly: 0_common.py, 1_sys1.py etc