cbbackupmgr remove 'cache' directory path from repo config

This task is more of a reminder to myself; after talking with members of the tools and kv team we decided that calling the S3 'cache directory' a cache directory is misleading. It's more of a staging area (and should be named as such).

We should remove the 'obj-cache-dir' value from the repository config and make it a required argument (which can also be supplied as an environment variable). This is because this is local config and doesn't really belong in the archive e.g. (we don't store the local git config in the repository).