Upgrade k8s to 1.15.4-k3s.1

Move download to Rancher repo

Upgrade CoreDNS to v1.6.3

install.sh: fix openrc service depend in heredoc

install.sh: Add wget fallback if curl isn't available

This change sets up the correct dependency on the `network-online`
service provided by openrc.

See:
- https://github.com/OpenRC/openrc/blob/0.39.2/service-script-guide.md#be-wary-of-need-net-dependencies
- https://github.com/OpenRC/openrc/blob/0.39.2/init.d/net-online.in#L18

Add back NodeRestriction

Switch to go modules

It has been removed as a part of #764 for no obvious reasons.

Fix #791

Add --kubelet-certificate-authority flag

Change-Id: I8acf9f0fc0115dd8a45027b1d7610b47aa7462d9

Fix CSI and update vendor

K8s 1.15

* bump traefik version

Changed SELinux/semanage error to fatal

If the install errors out on semanage not found, a file not found is
thrown as a 'file not found' for error. Updating to 'fatal' resolves
this as the script then exits as intended and throws an 'error'.

Enable metrics endpoint to Traefik

install.sh: Use -eq instead of = for integer comparison

Check for semanage command availability