Add mounts status to node_systemd_unit_state metric

[lionelwidmer]

Use-case:
Monitor that all defined mountpoints are currently mounted

Current situation:
No metrics expose the content of /etc/fstab, which make impossible to monitor that all mounts defined there are really mounted
On RHEL-like OSes, systemctl show the status of the mounts, but these are not being reported back by metric node_systemd_unit_state

Proposal:
Modify code for metric node_systemd_unit_state in order to include the mounts. Then metric node_systemd_unit_state
could be used to ensure all defined mounts are mounted

Example: I have unmounted /boot/efi on a server. Systemd reflects it perfectly well:
(0)LAB(VM)root@SERVER:/proc/net/stat# systemctl -a | egrep '.mount' | grep loaded | grep boot
boot-efi.mount loaded inactive dead /boot/efi
boot.mount loaded active mounted /boot