fix collaboration between --robot and --try-X

This effectively solves the long-standing issue of making `--robot` and `--try-X` work together as expected.

For example, to build Python v2.7.6 with the `goolf/1.5.14-no-OFED` toolchain when only an `ictce/5.5.0` version is available:

```
$ eb Python-2.7.6-ictce-5.5.0.eb --try-toolchain=goolf,1.5.14-no-OFED --ignore-osdeps --robot --dry-run
Couldn't import dot_parser, loading of dot files will not be possible.
== temporary log file in case of crash /var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/easybuild-Z_nF7M/easybuild-VdKThC.log
Dry run: printing build status of easyconfigs and dependencies
 * [x] /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/g/GCC/GCC-4.8.2.eb (module: GCC/4.8.2)
 * [x] /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/h/hwloc/hwloc-1.8.1-GCC-4.8.2.eb (module: hwloc/1.8.1-GCC-4.8.2)
 * [x] /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/o/OpenMPI/OpenMPI-1.6.5-GCC-4.8.2-no-OFED.eb (module: OpenMPI/1.6.5-GCC-4.8.2-no-OFED)
 * [x] /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/g/gompi/gompi-1.5.14-no-OFED.eb (module: gompi/1.5.14-no-OFED)
 * [x] /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/o/OpenBLAS/OpenBLAS-0.2.8-gompi-1.5.14-no-OFED-LAPACK-3.5.0.eb (module: OpenBLAS/0.2.8-gompi-1.5.14-no-OFED-LAPACK-3.5.0)
 * [x] /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/f/FFTW/FFTW-3.3.4-gompi-1.5.14-no-OFED.eb (module: FFTW/3.3.4-gompi-1.5.14-no-OFED)
 * [x] /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/s/ScaLAPACK/ScaLAPACK-2.0.2-gompi-1.5.14-no-OFED-OpenBLAS-0.2.8-LAPACK-3.5.0.eb (module: ScaLAPACK/2.0.2-gompi-1.5.14-no-OFED-OpenBLAS-0.2.8-LAPACK-3.5.0)
 * [x] /Users/kehoste/work/easybuild-easyconfigs/easybuild/easyconfigs/g/goolf/goolf-1.5.14-no-OFED.eb (module: goolf/1.5.14-no-OFED)
 * [ ] /var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/easybuild-Z_nF7M/ncurses-5.9-goolf-1.5.14-no-OFED.eb (module: ncurses/5.9-goolf-1.5.14-no-OFED)
 * [ ] /var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/easybuild-Z_nF7M/bzip2-1.0.6-goolf-1.5.14-no-OFED.eb (module: bzip2/1.0.6-goolf-1.5.14-no-OFED)
 * [ ] /var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/easybuild-Z_nF7M/zlib-1.2.7-goolf-1.5.14-no-OFED.eb (module: zlib/1.2.7-goolf-1.5.14-no-OFED)
 * [ ] /var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/easybuild-Z_nF7M/libreadline-6.2-goolf-1.5.14-no-OFED.eb (module: libreadline/6.2-goolf-1.5.14-no-OFED)
 * [ ] /var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/easybuild-Z_nF7M/Python-2.7.6-goolf-1.5.14-no-OFED.eb (module: Python/2.7.6-goolf-1.5.14-no-OFED)
== temporary log file /var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/easybuild-Z_nF7M/easybuild-VdKThC.log has been removed.
== temporary directory /var/folders/8s/_frgh9sj6m744mxt5w5lyztr0000gn/T/easybuild-Z_nF7M has been removed.
```