use --fix-missing to avoid Unable to find package git error on aws

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
standardize on apt sources.list for vbox and aws

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
updated Berksfile.lock

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
added build_identifier for easier box versioning

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
formatting

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>