This commit adds debug output whenever libostree reads GPG keys, which
can come from different locations in the file system. This is especially
helpful in debugging "GPG signatures found, but none are in trusted
keyring" errors, which in my case was caused by OSTree looking in
/usr/local/share/ostree/trusted.gpg.d/ rather than
/usr/share/ostree/trusted.gpg.d/.
Closes: #1241
Approved by: cgwalters
