hosts/gentoo: fix nonstandard path to systemctl command

Starting Vagrant on my Gentoo machine with systemd outputs a "command not found" message:

```
[default] Exporting NFS shared folders...
[default] Preparing to edit /etc/exports. Administrator privileges will be required...
Password: 
sudo: /usr/sbin/systemctl: command not found
[default] Mounting NFS shared folders...
[default] Running provisioner: shell...
```

That's because /usr/sbin/systemctl is not a standard path to the systemd command in Gentoo. The patch provided fixes this by using the /usr/bin/systemctl path instead.

NOTE that after making the systemctl fix there are more path issues from plugins/hosts/linux/cap/nfs.rb where /usr/bin/exportfs is also a nonstandard path to exportfs in Gentoo:
```
[default] Preparing to edit /etc/exports. Administrator privileges will be required...
Password: 
nfsd.service - NFS server
sudo: /usr/bin/exportfs: command not found
[default] Mounting NFS shared folders...
```

Gentoo uses /usr/sbin/exportfs as the standard path to exportfs. Another issue with nfs.rb are the references to "/etc/init.d/" which we can't rely on for systemd users. However, Vagrant still runs on my machine even with the exportfs and "/etc/init.d/" issues.