Change the 'savanna-db' scheme to 'internal-db'

The url scheme indicating internal store for job binaries has
been changed from 'savanna-db' to 'internal-db'. All of the constant
strings in the code referencing 'savanna-db' have been changed.

Partial-Implements: blueprint savanna-renaming-service
Change-Id: I94baab611570679bfff817bde2e5400a63d364ca