Fix typo in error message

Change-Id: Ia37ec318329dc527b30a1835e4a52ace14b2bda3
Fix nova boot --num-instances option checking

'args.num_instances' should be checked by None because of '0 == False'.
This patch fixes the checking.

Fixes bug 1151787

Change-Id: Iede860caa5c2d03d49471f375d687c3e40102372
Merge "Fix typo in error message"