Clean stemcell-building VM BOSH during provisioning

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>

Conflicts:
	bosh-stemcell/Vagrantfile
Upgrade stemcell-building vm to trusty

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Moving stemcell stages to make bosh_clean/harden be after package installation (/etc/resolv.conf is removed)
Fixing downloaded_release_path to be /tmp inside stemcell builder vm
Fixing drbd for trusty stemcell builds
Adding python package to trusty stemcell (allows git in v172 to compile properly)