The --as flag displayed (default "bot"), (default "user"), or
(default "auto") in help text, but ResolveAs() never uses the cobra
default — it resolves identity via credential config and auto-detect.
The displayed default misled users into thinking a fixed identity was
used when --as was omitted.
Set cobra default to empty string so no (default ...) suffix appears.
Also remove "auto" from visible options since --as auto is equivalent
to omitting --as entirely.
Change-Id: I51ba550a6697eb3675a29f5cee4d0010e0a1cc16