Fixed bin/cpi output for create_stemcell to be a cid string

- other methods may return ints or maps

Signed-off-by: Karl Isenberg <kisenberg@pivotal.io>
Bumped bosh-agent godep

Signed-off-by: Karl Isenberg <kisenberg@pivotal.io>
Added BOSH_PACKAGES_DIR & BOSH_JOBS_DIR

- when running bin/cpi create_stemcell

Signed-off-by: Karl Isenberg <kisenberg@pivotal.io>