Document a little the concerns for operators.

Change-Id: I2429371d3c5d931d46674c4e46d99599fda9f42e
Add ability to add extra apt keys

Adds an environment variable DIB_ADD_APT_KEYS pointing to a folder of
apt repo GPG keys. Each key will be verified and installed with apt-key
to the apt keyring.

Change-Id: I4b15347a76aa3241e741931bb94f17598641ae42
Merge "Document a little the concerns for operators."
Merge "Add ability to add extra apt keys"