make sure only first path in list of build/install/source paths is attempted to be created

This fixes a bug introduced in #874.

Using a list of source paths, only the first path should be created if it doesn't exist. The other paths are considered read-only, and are simply ignored when they don't exist. Trying to create them may fail because of permission issues, e.g.:

with:
```bash
export EASYBUILD_SOURCEPATH=/tmp/new/source/path:/foo/bar
```

```
    config.init(eb_go.options, eb_go.get_options_by_section('config'))
  File "/Users/kehoste/work/easybuild-framework/easybuild/tools/config.py", line 372, in init
    create_dir(key, directory)
  File "/Users/kehoste/work/easybuild-framework/easybuild/tools/config.py", line 357, in create_dir
    _log.error("Failed to create directory %s: %s" % (dirname, err))
  File "/Users/kehoste/work/easybuild-framework/easybuild/tools/build_log.py", line 104, in error
    raise EasyBuildError(newMsg)
EasyBuildError: "EasyBuild crashed with an error (at easybuild/tools/config.py:357 in create_dir): Failed to create directory /foo: [Errno 13] Permission denied: '/foo'"
```