Option for including access token to VM

[tdviet]

During initialization, client tools on VMs may need access tokens to access various services. It is important to have option to include the access token that is used for creating the VM, to the VM (via cloud-init) so users don't have to copy the access token manually.

That has been agreed in the discussion via fedcloud mailing list that we will follow WLCG suggestion (https://zenodo.org/record/3937438):

1. If the BEARER_TOKEN environment variable is set, then the value is
   taken to be the token contents.
2. If the BEARER_TOKEN_FILE environment variable is set, then its value is
   interpreted as a filename. The contents of the specified file are taken
   to be the token contents.
3. If the XDG_RUNTIME_DIR environment variable is set*, then take the
   token from the contents of $XDG_RUNTIME_DIR/bt_u$ID **.
4. Otherwise, take the token from /tmp/bt_u$ID . If a potential token is
   found at a step, then the discovery implementation MUST strip all
   whitespace on the left and right sides of