Adds support for a jwt token in a file. Simply reads the file and sends
the read in jwt along to the vault login.
It also supports a legacy mode with the jwt string being passed
directly. In which case the path is made optional.
Does the required dance with the local HTTP endpoint to get the required
data for the jwt based auth setup in Azure. Keeps support for 'legacy'
mode where all login data is passed on via the auth methods parameters.
Refactored check for hardcoded /login fields.
John Eikenberry
Chris Thain