Cache's notice messages get overwritten

[nishakm]

Describe the bug
When tern pulls data from the cache and saves data to the cache, it overwrites the notice messages that used to exist, thus losing the messages that got created when it processes an image the first time around

To Reproduce
Steps to reproduce the behavior:

1. Run tern -cl report -i <image>
2. Look at cache.yml to see messages about base OS and commands used to get the metadata
3. Run tern -l report -i <image>
4. Look at cache.yml to see message "loading from cache" but no information about the OS and commands used to get the metadata

Expected behavior
Using the cache should not overwrite the original messages.