fixes CC-595: change serviced script log format to be ~/serviced/script-...

...2014-12-18-083236.log

DEMO:
```
# plu@plu-9: serviced -v=1 script run -n abc.txt
Logging to logfile: /home/plu/serviced/script-2014-12-18-083327.log
I1218 08:33:27.582876 26743 script.go:108] syscall.exec unix script with command: [/usr/bin/script --append --return --flush -c serviced -v=1 script run -n abc.txt /home/plu/serviced/script-2014-12-18-083327.log]
Script started, file is /home/plu/serviced/script-2014-12-18-083327.log
I1218 08:33:27.618165 26755 script.go:115] runScript filename:abc.txt &{ServiceID: DockerRegistry: NoOp:false TenantLookup:<nil> Snapshot:<nil> Commit:<nil> Restore:<nil> SvcIDFromPath:<nil> SvcStart:<nil> SvcStop:<nil> SvcRestart:<nil> SvcWait:<nil>}
open abc.txt: no such file or directory
Script done, file is /home/plu/serviced/script-2014-12-18-083327.log
```