Use prefix trie lookup for IPv4 by default.

Unless otherwise configured, the prefix trie lookup is enabled for
IPv4 destination and source address fields.  A new keyword "none" is
accepted as the value of "prefixes" in the OVSDB Flow_Table column.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>