Add a new configuration option 'channel' to support deployment
of vault from channels other than stable.
This superceeds the layer option which is dropped as part of this
review.
Channel config option changes are honoured, but vault may not be
restarted depending on whether the local vault service has been
unsealed.
Change-Id: I845cd8db3a7e80155f2b485bb3f07934476996f2
6d593a01c0