Utilize keylength puppet conf value for generating SSL cert keys

The puppet.conf file supports a setting called keylength which controls the bit length for keys generated for use with SSL certificates:

https://docs.puppetlabs.com/references/latest/configuration.html#keylength

When generating keys for SSL certificates, the Puppet Server CA always uses 4096 bits, which is the default for the keylength setting. For backward compatibility, the Puppet Server CA should make use of the keylength setting to control the bit length of the keys it generates.