Implement diskmanager worker
worker/diskmanager: add tests
api/diskmanager: add tests
apiserver/diskmanager: add tests
Fix worker/diskmanager interface; fix vet warning

Make version.OSType a Stringer so it can be used as
argument to %s in printf functions.
cmd/jujud: remove diskmanager worker

Will add this back in in a follow-up.
storage: add package docs, fix comments
state/multiwatcher: move remaining mutliwatcher related types
tests pass
remove unused types
remove vestigial type decl
tests pass
Address review comments
update to new utils hash
worker/diskmanager: address review comments
responding to review changes
Set/fix best known facade version for DiskManager
Merge pull request #1120 from davecheney/213-set-strings-refactor

all: update dependencies.tsv and callers to set.Strings change

Update all uses of set.Strings, for #86/#87 from juju/utils

TODOs will be addressed in a followup PR

There are no uses of `set.Tags` in the codebase.
Merge branch 'master' of https://github.com/juju/juju into 212-state-multiwatcher-info-types
responding to review feedback