End-to-end test for Egress NAT

[ymmt2005]

This PR contains necessary changes to run Egress NAT feature and adds end-to-end tests for the feature.

• The base container image is changed from scratch to Ubuntu. This is necessary to run modprobe and iptables.
• coil-controller is granted to access Pods. This is necessary because coil-controller needs to grant the same privilege to Egress pods.
• coil-egress is run as privileged. This is necessary to edit sysctl files under /proc/sys/net.
• coil-egress should use the name of Egress resource to find client pods instead of the name of itself.