Override failing package signature checks

Override failing package signature checks

The RPMs built by teuthology's buildpackages task are not signed and
after a recent zypper update the install task started to fail with

File 'repomd.xml' from repository 'ceph-rpm-under-test' is unsigned, continue? [yes/no] (no): no
Error building the cache: [ceph-rpm-under-test|http://149.202.175.91/ceph-rpm-sle12-x86_64-basic/sha1/3804e807353c9d125753b1cf4f6405f79db83d4e/x86_64] Valid metadata not found at specified URL

Signed-off-by: Nathan Cutler <ncutler@suse.com>