Replace -proxy-mode=<bool> with -proxy=<enum>

The proxy flag really needs three values, not just true/false. Establish the flag as an enum now rather than after v0.5-rc is cut to prevent unnecessary churn. Once the `activeSize` cluster config is actually supported, we can implement the "fallback" proxy mode.
we should just implement this as a new flag type, ala: https://github.com/coreos/etcd/pull/1108/files#diff-7cd159a8d9276a11b8d5b9c888d1ea24R15