[CBM] Can't take a backup of a cluster where there isn't a full set of active vBuckets

What's the issue?
It's currently not possible to create a backup of a cluster where it doesn't have a full set of active vBuckets, this is because 'cbbackupmgr' will try (and fail) to get all the expected sequence numbers.

Error backing up cluster: failed to get sequence numbers: exhausted retry count after 5 retries

What's the fix?
When we get the vBucket count (which is used when fetching sequence numbers) we should use the number of active vBuckets, not the number of expected vBuckets.