Change the default value of knox.token.impersonation.enabled

Currently the default value of knox.token.impersonation.enabled is true. That is, if this config is not declared in KNOXTOKEN service parameters, token impersonation is enabled.

We need to change this default value to false. This way customers will bear in mind that enabling token impersonation itself is not enough, they also need to configure the required token proxyuser config. See details here.