shell command defaults to /bin/bash when no command is specified

Demo:
```bash
 {develop} [...]/control-center/serviced$ serviced service shell 9dlm0bhm061714it9e8thqocg
I1016 12:13:12.066151 29379 server.go:341] Connected to the control center at port 10.87.111.175:4979
I1016 12:13:12.178677 29379 server.go:435] Acquiring image from the dfs...
I1016 12:13:12.179391 29379 server.go:437] Acquired!  Starting shell
bash-4.2# Trying to connect to logstash server... 127.0.0.1:5042
Connected to logstash server.

bash-4.2# exit
```