Remove service _user, _password, _tenant from context

Remove service _user, _password, _tenant from context

These are not currently used anywhere in heat-engine, and I'm
not comfortable with the service password going over the (controller)
network.

If heat-engine needs access to these in the future then they can always
be configured in heat-engine.conf, or the operation can be performed
in the api and the results put into the context.

Change-Id: I81aab6b5a2c9aa7d6412531f70108857cbfa637b