Use XDG/Freedesktop standard directories to store certificates
NM seems to store stuff in ~/.cert/nm-openvpn ...
It would sense to follow http://standards.freedesktop.org/basedir-spec/latest/ instead and use something like $XDG_DATA_HOME (I think that usually points to ~/.local/share/), and maybe with a folder name that is more indicative than "cert", something like $XDG_DATA_HOME/network-manager/certs/openvpn/ etc.