Build chroot tgz: add optional infrastructure argument
Build chroot tgz: add optional infrastructure argument

When building a chroot tgz via rake task, be able to optionally
specify the infraestructure target (vsphere, aws or openstack in
the future). Actually you can only build for vsphere.

Change-Id: I62e2519cab37cc39b281dbc6854d568681d061b6

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh