testsessionid parameter is not taken into account while running sgload

looks like # `testsessionid` parameter is not taken into account while running `sgload`.

`sgload` always generates a new uuid, regardless of my specifying a session id when wanting to reuse old users/channels/scenarios.

```
time ./sgload writeload --sg-url http://localhost:4984/db/ --batchsize 50 --numchannels 10 --numdocs 100 --numwriters 10 --createwriters --testsessionid foo4 --statsdenabled
INFO[06-15|12:27:09] Attempting to expose expvars port=9876
INFO[06-15|12:27:09] Setting loglevel level=info
INFO[06-15|12:27:09] Waiting for writers to finish numwriters=10
INFO[06-15|12:27:11] Created SG user username=writer-user-1-25f72080-3339-44f2-80ac-497fc432b870 channels=[*]
```

The user-provided value is always overridden by https://github.com/couchbaselabs/sgload/blob/master/cmd/common.go#L86

```
loadSpec.TestSessionID = sgload.NewUuid()
```