Adding a file-based fakezk-based topo for vtgate.

Will use it in various unit tests to fake a topology.
Topo integration with vtgate. Also added mechanism to use topo with zkocc.
Topo integration with vtgate. Also added mechanism to use topo with zkocc.
Improved zkocc_test to compare vtgate zk, zkocc implementation. Pruned vttopo everywhere.
Merge branch 'master' of github.com:youtube/vitess
misc fixes

noOutput *string -> UnusedResponse
tabletmanager,py breaks up tuple return from vtgate launch
Receive transaction id through TransactionInfo struct: The reason
why this worked before is because bson wraps simple types as structs.
But we want to move away from such usage.
missed couple more string -> UnusedResponse
add logging for relevant errors