Support setting environment variables in platform_configuration's yaml files

Currently the yaml files contain the same data as the old .txt format we used. It contains data about used tier1 iamge,  compiler, coin flags, and configuration flags.

In for instance ICC's case, we need to modify the LD_LIBRARY_PATH and PATH when using that compiler. Those can't be provisioned, as it would change the paths also when we build with GCC and don't use ICC.

So, let's add support for setting environment variables inside the yaml file.

It also could support extrapolating the environment variables, so that we can set LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:/foobar and it would just add the current path, but not replace. Or, MAKEFLAGS=-j${NUMBER_OF_PROCESSORS} for example.