rose mpi-launch breaks if ROSE_LAUNCHER contains spaces

[dpmatthews]

Rose 2019.01.5:

$ export ROSE_LAUNCHER="mpiexec"
$ rose mpi-launch -v true
[INFO] exec /path/to/mpiexec -n 1 /usr/bin/true
$ export ROSE_LAUNCHER=" mpiexec"
$ rose mpi-launch -v true
[INFO] exec /path/to/mpiexec -n 1 /usr/bin/true
$ export ROSE_LAUNCHER="time mpiexec"
$ rose mpi-launch -v true
[INFO] exec /usr/bin/time /path/to/mpiexec /usr/bin/true
...

Rose 2.0rc2:

$ export ROSE_LAUNCHER="mpiexec"
$ rose mpi-launch -v true
[INFO] exec /path/to/mpiexec -n 1 /usr/bin/true
$ export ROSE_LAUNCHER=" mpiexec"
$ rose mpi-launch -v true
[FAIL] ROSE_LAUNCHER:  mpiexec: command not found
[FAIL] exit 1
$ export ROSE_LAUNCHER="time mpiexec"
$ rose mpi-launch -v true
[FAIL] ROSE_LAUNCHER: time mpiexec: command not found
[FAIL] exit 1