repo: Don't set GPG engine executable path

The instructions one finds on the internets are apparently wrong, we
really need to keep the default here, since gpgme uses it to actually
find the helper binary it runs.

This fixes the GPG tests for me on EL7 at least.
tests: Fix up GPG tests for more strict EL7 GPG

These GPG tests were failing for me on EL7 - it appears to be because
we had only one directory for both private and public keys, and we
were giving that to ostree for verification, which passed them onto
gpgv.

In EL7 beta at least, gpgv now barfs if it finds a private key where
it is just expecting to find public keys.

Fix this by splitting out the public trusted directory from the
private key directory.  Except now for signing, we still need the
public key there, so symlink it.  Whee!
pull: Remove a duplicate hash table

Not sure why we had two...perhaps the code originally had them
separate.
deltas: Add a timestamp to delta metadata
repo: Split generic GPG commit verification out into helper

This will be used for a future commit which GPG verifies static
deltas.