Adding an InitTablet feature to vttablet.

It only covers a subset of what the real InitTablet does,
but should be enough to not have to initialize tablets before starting them.
@enisoc @aaijazi 
The flags will work to tell vttablet itself, but the Tablet class in test/tablet.py also needs to know these things. Usually the Tablet class remembers them when you call Tablet.init_tablet(), but here he's removing the call to init_tablet().