nrpe and nrpe-external-master charms

[Liam Young]

tl;dr I would like to replace the trusty nrpe charm in the charmstore 
with my rewrite (lp:~gnuoy/charms/trusty/nrpe/services-rewrite) which 
combines the nrpe-external-master and nrpe charms. Bug #1430732 tracks 
its submission.

As the README in the precise nrpe-external-master charm states it was 
always intended that one day it would be merged with the nrpe charm, so 
I have taken the opportunity to do just that. The old charms were 
written in a mixture of bash and python and contained a workaround for 
old pyjuju limitation which were fixed long ago. I have written the new 
charm in python using the services framework and added amulet tests.

The options to the charm are the same as the nrpe-external-master charm 
with the addition of the 'monitors' option, which the old nrpe charm 
implemented, this allows the user to dynamically add predefined checks. 
The new charm has an export_nagios_definitions option which triggers the 
charm to write out nagios config fragments and allow them to be collect 
via rsync (the old nrpe-external-master behaviour).

The new charm has the same monitors interface and supports three 
different interfaces with a principle charm: nrpe-external-master, 
local-monitors and juju-info. I've left the nrpe-external-master 
interface support in for compatibility with existing bundles but I don't 
think it adds any value and we should deprecate that interface type.

I've tested upgrading from the old nrpe charm to the new one and also 
switching from the old nrpe-external-master charm to this one.

Regards,

Liam Young