copy_nrpe_checks doesn't verify the directory to copy from.

[ajkavanagh]

Due to LP:#1796830 it turns out that the copy_nrpe_checks() function assumes that charmhelpers will always be installed in the hooks directory (which was probably a reasonable assumption). However, certain issues around converting PY2 charms to PY3 charms has meant, that for some charms, charmhelpers is better placed in the CHARMDIR. This has the unfortunate effect of breaking copy_nrpe_checks.