newt - Fix handling of private github repos.
newt - Fix handling of private github repos.

MYNEWT-804 Access to private repos using personal access tokens is broken
MYNEWT-805 Better support for private repos

The current handling of private github repos is broken in a few ways:

1. Only used for retrieval of repository.yml; manual login+password
   entry required for cloning.
2. Authentication tokens were handled incorrectly.  These should be
   handled just like passwords.

This commit provides the following fixes:

1. Use configured login and password for cloning as well as
   repository.yml retrieval.
2. Remove 'token' configuration item ('password' works for tokens as
   well).
3. Add 'password_env' configuration item.  This specifies the name of an
   environment variable containing the password for the private repo.
   This setting is only used if 'password' is empty.

apache/incubator-mynewt-newt,apache/incubator-mynewt-newt