[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).