testing: never generate instance without memory or CPU
testing: never generate instance without memory or CPU

These are not realistic test cases, so we do not have
to worry about them. Moreover, allowing instances without
any memory usage will make example cluster generated by
iterated allocation fail, as the ahead-of-time estimate
of the number of iteration steps will result in a division
by 0.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

andir/ganeti,leshchevds/ganeti,ganeti/ganeti,bitemyapp/ganeti,yiannist/ganeti,andir/ganeti,onponomarev/ganeti,leshchevds/ganeti,ganeti/ganeti,mbakke/ganeti,leshchevds/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti/ganeti,bitemyapp/ganeti,onponomarev/ganeti,andir/ganeti,yiannist/ganeti,yiannist/ganeti