PKCS11: fails with CKR_ATTRIBUTE_READ_ONLY in unwrap_key function

[Jan Stodt]

Setting the attribute CKA_EXTRACTABLE manually for key unwrapping results in
 CKR_ATTRIBUTE_READ_ONLY error. This error occurs due because that CKA_EXTRACTABLE
 is already set by PKCS#11 automatically, as specified in the PKCS11 standard.

 "The CKA_EXTRACTABLE attribute is by default set to CK_TRUE." [1]

Tested with openCryptoki v3.4.1 ICA token, but should be a general issue since the Barbican PKCS#11 plugin does not follow the PKCS11 v2.40 standard.

How to reproduce:
- Start Barbican
- Store a secret

Appears after fix for wrong GCM header has been applied [2].
Otherwise we do not even get this far.

 [1] http://docs.oasis-open.org/pkcs11/pkcs11-base/v2.40/os/pkcs11-base-v2.40-os.pdf
 p. 132 - Section 5.13
[2 ]https://review.openstack.org/#/c/483378/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/483388

[OpenStack Infra (hudson-openstack)]

Change abandoned by Douglas Mendizábal (<email address hidden>) on branch: master
Review: https://review.openstack.org/483388
Reason: Abandoning patch due to lack of activity for months. Feel free to re-submit if needed.

[Grzegorz Grasza (xek)]

Closing out bugs created before migration to StoryBoard. Please re-open if you are of the opinion it is still current.