use image weaveworks/ubuntu instead of centos:7 in command examples

[silenceshell]

The centos:7 image does not include ssh server, so if I run:

	$ ignite run centos:7 \
		--interactive \
		--name my-vm \
		--cpus 2 \
		--ssh \
		--memory 2GB \
		--size 10G

As the image has no ssh server, I cannot ssh to the vm. Here is the steps.

# ignite  run centos:7 --interactive --cpus 1 --memory 4GB --ssh --name vm-centos
.....
# ignite ps
VM ID			IMAGE				KERNEL					SIZE	CPUS	MEMORY	CREATED	STATUS	IPSPORTS	NAME		vm-1
ef0bb86f13f9bcd5	centos:7			weaveworks/ignite-kernel:4.19.47	4.0 GB	1	4.0 GB	14s ago	Up 14s	172.18.0.11		vm-centos
# ignite ssh vm-centos
ssh: connect to host 172.18.0.11 port 22: Connection refused
WARN[0000] SSH command terminated

I think the weaveworks/ignite-ubuntu is a better choice, as mentioned here.

[luxas]

Awesome, thanks for fixing 💯