Port nrpe support into master branch (lp#1616525)

[Alvaro Uria]

A previous bug fixed nrpe support in systemd on a single Charm release (18.05):
https://bugs.launchpad.net/charms/+source/ceph-osd/+bug/1616525

This bug does the same but in the master branch, and it should be backported into 18.11.

[Alvaro Uria (aluria)]

Change proposed https://review.openstack.org/#/c/619012/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/619012
Committed: https://git.openstack.org/cgit/openstack/charm-ceph-osd/commit/?id=11e1dfc5656b2cd495d2f53049bf8c58aedb9614
Submitter: Zuul
Branch: master

commit 11e1dfc5656b2cd495d2f53049bf8c58aedb9614
Author: Alvaro Uria <email address hidden>
Date: Thu Aug 9 13:22:49 2018 +0200

    Support systemd nrpe check for ceph-osd@N units

    ceph-osd<->nrpe relation adds a single check that parses the service
    status of all the ceph-osd processes. The check supported sysv and
    upstart environments, but not systemd, which has been added.

    add_init_service_checks does support systemd but it would create a
    nrpe check per OSD (vs a single check for all OSDs)

    Change-Id: I34fc01365de6994c93a273f01a1e2278016d21ef
    Closes-Bug: 1804247
    Signed-off-by: Alvaro Uria <email address hidden>