Add "gpgkeypath" option to remotes

Add "gpgkeypath" option to remotes

For Project Atomic, we already have RPM signatures which use files in
`/etc/pki/rpm-gpg`.  It's convenient to simply bind the OSTree remote
configuration to those file paths, rather than having duplicate key
data.

This does mean that we need to parse the files for verification, so we
end up importing them into the verifier's temporary keyring, which is
a bit ugly, but it's what other projects do.

Closes: https://github.com/ostreedev/ostree/issues/573
Closes: #575
Approved by: giuseppe