cli: Disable abbreviation matching for options

Python's “optparse” module does option name prefix matching by default.
Since this can lead to confusing behaviour, e.g. by specifying “--force”
for a command which only has a “--force-multi” option, this patch
disables the prefix matching.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>