Add support for use of AppRole's for authentication to Vault; this
feature provides a more application centric approach to managing
long term access to Vault.
The functional tests exercise this integration with a restricted
policy which only allows access to the default 'secret' backend.
Change-Id: I59dfe31adb72712c53d49f66d9ac894e43e8bbad
Closes-Bug: 1796851
bc7f7a4c36