contint: monitor tmpfs mount on production slaves

The two production contint slaves (gallium and lanthanum) have a 512MB
tmpfs disk which might end up being totally filled.  In such a case, the
MediaWiki tests jobs bails out because they can not create a Sqlite
database there.

The directory should be cleaned up once build is complete.  Regardless
monitoring it is a good thing to have since it a disk full is disrupting
the CI workflow.

I have commented out critical => true, not sure whether ops should
receive pages for that.

Command tested on both hosts:

gallium:~$ /usr/lib/nagios/plugins/check_disk -w 20% -c 5% -e \
    -p /var/lib/jenkins-slave/tmpfs
DISK OK| /var/lib/jenkins-slave/tmpfs=331MB;409;486;0;512
gallium:~$

lanthanum:~$ /usr/lib/nagios/plugins/check_disk -w 20% -c 5% -e \
    -p /var/lib/jenkins-slave/tmpfs
DISK OK| /var/lib/jenkins-slave/tmpfs=79MB;409;486;0;512
lanthanum:~$

Bug: 69733
Change-Id: I812c89cb4348fcfdbf0df5db37e5c63164b4327a
Merge "contint: monitor tmpfs mount on production slaves" into production