unit tests: support softhsm2

[sklemer1]

The pkcs11 unit tests use softhsm as token. Recent Linux distros ship with softhsm2 which had some changes, notably a new config format, new env variables, different slot numbering. This tries to support it by keeping compatibility with softhsm1.

Please note:

• test_two_sessions now throws a CKR_USER_ALREADY_LOGGED_IN error which I think is right.
• My openssl version complained about the lower case 'cn' so I changed it.
• This PR builds upon pkcs11: Use 1st slot as default, not slot '0'. #43
• I don't have a softhsm1 setup so I can't test the downward compatibility.
• This PR works nicely with Switch to pyca/cryptography as crypto backend #41 which is the reason I did this in the first place...
• Because of this everything is just randomly thrown together until it worked -- code quality is rough.

[coveralls]

Coverage decreased (-0.4%) to 56.462% when pulling 71cd962 on sklemer1:softhsm2 into 982550b on IdentityPython:master.