Specify keySize for selfsigned certs

[mrxz]

The selfsigned library uses a default key size of 1024. Depending on the system's openssl configuration, this can lead to the following error, preventing Spoke from starting:

Error: error:140AB18F:SSL routines:SSL_CTX_use_certificate:ee key too small

It's a known issue with selfsigned (see https://github.com/jfromaniello/selfsigned/issues/33), but until they increase the default keySize, specifying it in the webpack.config.js file resolves this issue as well.