use tripwire when building stemcells

- only builds the tripwire database when the environment variable
  TW_LOCAL_PASSPHRASE is set
- stores a copy of tripwire.db in the stemcell, so it can be verified
  even if the running image is compromised

Change-Id: I5f8ff92a9d1aee9feb1061512f6e71f142149109
Merge "use tripwire when building stemcells"