return www-authenticate header on failed basic auth

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
only set permissions of files that exist

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
adding bosh_micro_go stage in go agent stemcells

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>