cannot run kata-runtime standalone with default path for bundle

[egernst]

Description of problem

https://github.com/kata-containers/runtime/blob/master/cli/run.go#L34

We should set a default to the current directory. Today this doesn't work.

cli.StringFlag{
			Name:  "bundle, b",
			Value: "",
			Usage: `path to the root of the bundle directory, defaults to the current directory`,
		},

To reproduce:

# create the top most bundle directory
mkdir /mycontainer
cd /mycontainer

# create the rootfs directory
mkdir rootfs

# export busybox via Docker into the rootfs directory
docker export $(docker create busybox) | tar -C rootfs -xvf -

runc spec

# run as root
cd /mycontainer
kata-runtime run mycontainerid