Don't use lsb_release

Using lsb_release to figure out what distro and release an image is
is redundant, as the same information is available in DIB_ vars.
Further, it causes errors when creating new rhel7/centos7 images
like this: http://paste.openstack.org/show/144729/

Change-Id: I213ee581b8abfe119ffe3b783e6a3236e58e3e47