[stemcell] Increase OpenStack root partition to 10Gb

When OpenStack boots a new stemcell it resizes the virtual disk size, but
it doesn't resize partitions inside the image. That means that we can have
a virtual root disk size of 10Gb but only available the size set at the
stemcell builder (actually 2Gb).

In this commit we are increasing the virtual disk size and the root
partition size to 10Gb. So whatever OpenStack flavor we're using, we
can assure that the root partition will always have 10Gb. This will help
us to deal with flavors that doesn't have ephemeral disk, as now we can
use the root partition to store the BOSH agent data.

Note: increasing the root partition doesn't affect the size of the stemcell
archive.
[openstack_cpi] Allow flavors without ephemeral disk
[cli] Allow 'vms --vitals' command without ephemeral disk
[agent] Allow no ephemeral disk for OpenStack
[openstack_cpi] Select attachment device name based on server flavor