Default to base stemcell 0.7.0 [fixes #123]

This is until there are new stable stemcells (1.5.0)
using best_bosh_stemcell_version to select stemcell to default to
revert to latest_bosh_stemcell_version to check what's currently in bosh
Ignore stemcells with pre in their name
grep -V prints version info; -v is what we need