* add TencentCloud COS backend for remote state
* add vendor of dependence
* fixed error not handle and remove default value for prefix argument
* get appid from TF_COS_APPID environment variables
In order to support free organizations, we need a way to load the `remote` backend and then, depending on the used offering/plan, enable or disable remote operations.
In other words, we should be able to dynamically fall back to the `local` backend if needed, after first configuring the `remote` backend.
To make this works we need to change the way this was done previously when the env var `TF_FORCE_LOCAL_BACKEND` was set. The clear difference of course being that the env var would be available on startup, while the used offering/plan is only known after being able to connect to TFE.
This work was done against APIs that were already changed in the branch
before work began, and so it doesn't apply to the v0.12 development work.
To allow v0.12 to merge down to master, we'll revert this work out for now
and then re-introduce equivalent functionality in later commits that works
against the new APIs.
Li Kexian