Use __opts__.get

This switches `tests/saltsh.py` to using `__opts__.get('id')` rather than `__opts__['id']`.

Problem:

When trying to use saltsh.py, on a fresh box 'id' was not present. This resulted in saltsh.py failing:

```
root@qa01:~/salt/tests# python saltsh.py
Traceback (most recent call last):
  File "saltsh.py", line 132, in <module>
    main()
  File "saltsh.py", line 101, in main
    salt_vars = get_salt_vars()
  File "saltsh.py", line 83, in get_salt_vars
    __opts__['id'],
KeyError: 'id'
```

Version data
```
root@qa01:~/salt/tests# salt --version
salt 2014.1.0-733-g2649cbc
```