Fail if running outside of netty be default

[spencergibb]

Gateway only supports netty at the moment, see #145.

Gateway should fail if running in tomcat, jetty or undertow until the above issue is resolved.

[spencergibb]

This should not preclude tests using wiremock.

[thakurvikram]

I am facing the same issue while deploying spring cloud gateway code on tomcat.
Error: failed to introspect class GatewayAutoConfiguration$NettyConfiguration

Is this fixed in any version?