nrpe.py doesn't handle nagios_hostname_type

[JuanJo Ciarlante]

nrpe charm supports nagios_hostname_type to be either
'host' (use actual hostname) or 'unit' (derive hostname from unit_name).

Currently if you deploy e.g. nova-compute then later relate to
nrpe with nagios_hostname_type='host' you'll end with e.g.:

$ ls /var/lib/nagios/export/*.cfg
/var/lib/nagios/export/host__geneva.cfg
/var/lib/nagios/export/service__bootstack-staging-nova-compute-1_check_libvirt-bin.cfg
/var/lib/nagios/export/service__bootstack-staging-nova-compute-1_check_nova-compute.cfg

$ /var/lib/nagios/export
$ egrep host_name *cfg
host__geneva.cfg: host_name geneva
service__bootstack-staging-nova-compute-1_check_libvirt-bin.cfg: host_name bootstack-staging-nova-compute-1
service__bootstack-staging-nova-compute-1_check_nova-compute.cfg: host_name bootstack-staging-nova-compute-1

NOTE in above 3 lines the mismatching host_name (will break nagios)