shared: Add GPG keys dir

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.build_helper: Fix problem with verifying signed tags

Michael pointed out that there are 2 problems with the
current implementation of the signed tags verification:

1) If the tag is signed by Malicious.Hacker@somewhere
and you trick gpg into downloading this key, it will still
happily accept the tag as long as it's in the server.
2) If one runs this test from her/his account, it will add
random stuff to her/his .gnupg directory.

In order to bypass chain of trust issues, the user will
specify directly which keys are allowed, those keys will
be copied to a special shared location on virt test
(shared/gpg), the environment variable GNUPGHOME will be
set to a temporary directory, the specified key will be
imported and then the tag will be verified.

CC: Michael S. Tirskin <mst@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>