Add a gnupg keyring for release tag signature verification

Add a gnupg keyring containing just the public part of the keypair used
to sign release tags.

Also change the githelper script to use the keyring in the git repo
instead of the one in the caller's home directory.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
Merge remote-tracking branch 'origin/2.6' into 2.7