vbox: Remove --hostonlyadapter1 option

This option is needed only in --hostonly mode not in --nat mode.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Merge pull request #33 from asias/nic

vbox: Remove --hostonlyadapter1 option
pull: Download hypervisor specific image

Fix 'capstan pull' and related commands to only download the hypervisor
specific image that is going to be used from S3 repository. Github
repositories are still pulled fully because there's no way to partially
clone a git repository.

Fixes #19.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Add gce support

Signed-off-by: Asias He <asias@cloudius-systems.com>