gce: Support DeleteVM

Changing the name and arg of vmDelete to DeleteVM which will be used by
`capstan delete` cmd.

Signed-off-by: Asias He <asias@cloudius-systems.com>
vbox: Change DeleteVM to be used by capstan delete

After this, instance is not deleted when stopped.

Signed-off-by: Asias He <asias@cloudius-systems.com>
vmw: Change DeleteVM to be used by capstan delete

After this, instance is not deleted when stopped.

Signed-off-by: Asias He <asias@cloudius-systems.com>
qemu: Change DeleteVM to be used by capstan delete

Signed-off-by: Asias He <asias@cloudius-systems.com>
cmd/delete: Introduce capstan delete cmd

This cmd is used to delete an instance on the local disk or in the
cloud.

Signed-off-by: Asias He <asias@cloudius-systems.com>
cmd/stop: Fix typo in usage message

It is stop not info.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Merge pull request #68 from asias/delete

Implement capstan delete cmd