Update provisioning internals docs

 - Placement directives
 - Availability zone spread

Also, changed to use Markdown format.
apiserver: fix vet warning (missing arg)
wip
api/client: RetryProvisioning takes a Tag not a string

As it says on the tin.
dependencies.tsv: update juju/testing depdendency

Updated to fix a bug that showed up in #725
simplify, thanks axw
Merge pull request #725 from davecheney/196-api-client-retry-provisioning-takes-a-tag

api/client: RetryProvisioning takes a Tag not a string

As it says on the tin.
Merge pull request #727 from davecheney/197-update-juju-testing-dep

dependencies.tsv: update juju/testing depdendency

Updated to fix a bug that showed up in #725
Merge pull request #719 from axw/apiserver-tools-log-vet

apiserver: fix vet warning (missing arg)
Factor out cmd testing helpers.
Add an API facade for backups.
Add tests.
Add missing doc comments.
Wrap returned errors with errors.Trace().
Drop the superfluous "Backups" from some names.
Fix a typo.
impl -> backups.
Add the newMeta() test helper and use it.
Remove unnecessary XXX comments.
Use the more canonical form for an empty value.