pass inject dict down into connections

This currently doesn't change any functionality.  Not currently used but needed
to support using hostvars for connection settings.
allow setting SSH user / passwords from vars

Implements functionality to use the inject dictionary to find SSH usernames and
passwords.  This allows setting these values on the command line and in the
inventory (per host, group, etc).

SECURITY NOTE: Credentials in the inventory are not necessarily a good idea.
	       However, since the inventory is already not usually exposed,
	       this is a calculated risk.  When in doubt, just use SSH
               hostkeys.