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 -init_tags flag is a custom flag type (flagutil.StringMapValue). Custom flag types are added with flag.Var(). Unfortunately flag.Var() doesn't return any expression, so we can't call it in a variable initializer, as is the convention for built-in flag types like flag.String(). So for custom flag types, the convention is to call flag.Var() in init() instead.