Address verifier DeprecationWarning

The use of signer and verifier in cryptography has been
deprecated, and causes the following warning:

barbican_tempest_plugin/tests/scenario/barbican_manager.py:139:
DeprecationWarning: signer and verifier have been deprecated.
Please use sign and verify instead.

This patch replaces signer with sign.

Change-Id: Ia6358e2c4e84faa9d692a13b9b3aa1504407301b
1 file changed