[r=axwalk] provider/manual: set storage in SetConfig
We currently rely on EnableBootstrapStorage to
specify the storage mechanism to use for bootstrap
and sync-tools. This is insufficient in practice,
causing panics such as those in lp:1262764.
Instead, we inject a boolean attribute into the
environment config that is stored in state; this
attribute is false in the .jenv file, and true
in state. We use its value to decide whether to
use sshstorage (external) or httpstorage (internal).
A followup will eliminate BootstrapStorager.
Fixes lp:1262764
https://codereview.appspot.com/61480043/