commit: Store detached metadata in normal form

Otherwise corrupted variants may crash us.
pull: Verify commits with gpg signatures from detached metadata

This uses gpgv for verification against DATADIR/ostree/pubring.gpg by
default.  The keyring can be overridden by specifying OSTREE_GPG_HOME.

Add a unit test for commit signing with gpg key and verifying on pull;
to implement this we ship a test GPG key generated with no password
for Ostree Tester <test@test.com>.

Change all of the existing tests to disable GPG verification.