* many: change transactional flag to an option
Change the --transactional flag to an option called --transaction with
two possible values, "per-snap" and "all-snaps". The former is the
default, and makes updates/installs transactional per snap, while the
latter makes the API call transactional on the specified set of snaps,
so if one fails all the action is reverted.
* tests: adapt to change from --transactional to --transaction=...
* cmd/snap: tweak a bit description of transaction option
* daemon: do some additional checks on transaction option at the API level
* many: address some minor review comments