Handle nrpe one to many relationships
[thedac,r=mthaddon] Allow us to associate one nrpe-external-master subordinate with many primary charms
[dames,trivial] Add braces to relation variable
[dames,trivial] Add empty relation variable for non-relation hooks
Use JUJU_UNIT_NAME for relation in filename if no relation exists
[rfinnie,r=dames] Allow charms without an nrpe-external-master hook to get the basic checks
[thedac,r=mthaddon] Allow one nrpe-external-master to many service charms