Tests: updated ssl_engine_keys.t test to use SoftHSM v2.

Notably, this implies not using slots to identify keys, since
"softhsm2-util --init-token" automatically reassigns initialized token
to a new slot.

Additionally, the "-config" option of is no longer used when generating
certificates, as in OpenSSL 1.1.0 and later it conflicts with the
configuration file provided via OPENSSL_CONF and results in "conflicting
engine id" errors.