[WIP] Systemd Service Bin Paths

[djtaylor]

This address the following issue on Ubuntu 16.04.2:

The path to yas3fs is /usr/local/bin/yas3fs on my system, which is not taken into account by the systemd template and config parameters.

Jun 17 11:14:46 demo-openstack1 systemd[1]: s3fs-s3homes.service: Service hold-off time over, scheduling restart.
Jun 17 11:14:46 demo-openstack1 systemd[1]: Stopped S3-backed FUSE filesystem.
Jun 17 11:14:46 demo-openstack1 systemd[1]: Started S3-backed FUSE filesystem.
Jun 17 11:14:46 demo-openstack1 systemd[2120]: s3fs-s3homes.service: Failed at step EXEC spawning /usr/bin/yas3fs: No such file or directory
Jun 17 11:14:46 demo-openstack1 systemd[1]: s3fs-s3homes.service: Main process exited, code=exited, status=203/EXEC
Jun 17 11:14:46 demo-openstack1 umount[2122]: umount: /Users: not mounted
Jun 17 11:14:46 demo-openstack1 systemd[1]: s3fs-s3homes.service: Control process exited, code=exited status=32
Jun 17 11:14:46 demo-openstack1 systemd[1]: s3fs-s3homes.service: Unit entered failed state.
Jun 17 11:14:46 demo-openstack1 systemd[1]: s3fs-s3homes.service: Failed with result 'exit-code'.

This also converts the systemd configuration to use /usr/bin/env to discover bin files. This is more compatible then hard-coded paths.