Tests: unbreak ssl_password_file.t with upcoming OpenSSL 1.0.2. The 512-bit keys, as generated by default by "openssl genrsa" in older versions, are rejected by OpenSSL library built from master branch (and upcoming OpenSSL 1.0.2). This brokes the test if the "openssl" binary is old (e.g., system one), but nginx is compiled against new OpenSSL. Fix is to explicitly generate 2048 bit keys. This is also consistent to what we generate in other places.