Apply SslBundleProperties.Key property to PEM keystore and not to the truststore

[philwebb]

Once #38124 is in place we can apply an alias and password only to specific stores. For our PEM support, it makes sense to apply the <bundle>.key.alias and <bundle>.key.password properties to just the keystore. It could be argued that it's a bug in 3.1 that we apply the <bundle>.key.alias to both the truststore and keystore, but it's going to be hard to fix in that branch so I think we can target this to 3.2 only.