Service token support [zed]

[freyes]

Backports of pull requests listed below to get support for Service Tokens in charm-helpers' stable/zed branch.

• https://github.com/juju/charm-helpers/pull/740
• https://github.com/juju/charm-helpers/pull/757

[freyes]

@drencrom , if you could review this patchset would be great, I think I got all the patches, but in case you are aware of any other that should be backported let me know

[drencrom]

@drencrom , if you could review this patchset would be great, I think I got all the patches, but in case you are aware of any other that should be backported let me know

LGTM, these two are the relevant patches.