new bundle format: default value for num_units

[roger peppe]

With the new bundle format [1] we recently discovered there is a problem
with respect to the service num_units field - bundles migrated to the
new format would always gain a num_units: 1 field which is wrong when
the service uses a subordinate charm.

We'd like to propose that when the num_units field is omitted from a
service, it will imply 0 units (or n subordinate units when service is
a subordinate).  This is a change from the original format where omitting
num_units implies 1 unit for non-subordinate charms.

Doing this means we can show an accurate non-subordinate unit counts
for a bundle without needing to fetch the charms that it uses (in the
old format, it's not possible to tell whether a service has 1 unit or
an indefinite number).

Theoretically it's possible for a charm to change from subordinate to
non-subordinate between revisions so even if we fetch the charms, that
does not necessarily imply the count remains the same in the future.

It also simplifies a bunch of the code :)

Note that when migrating from the old format, we would omit the
num_units field for subordinate charms, and add it explicitly
set to 1 for non-subordinates. Therefore, the proposed change of
defaulting to 0 only affects usage of the new format.

My question, for those of you that use bundles a lot, would it be a
problem to have to be explicit about the number of units in a service?

  cheers,
   rog.

[1] https://docs.google.com/document/d/1SF8hTBi6oVbki8V__beNij6wnQU-5cm6PZsy5gf0j_Y