Follow-up on commit ec16ee7 .

…s not exist

or is empty.

Raise an `Exception` if `--galaxy_root` specifies an existing directory and
`--install_galaxy` is set.

Remove unused `_shed_tool_conf()` function and
`FAILED_TO_FIND_GALAXY_EXCEPTION` constant.

to not rebuild the Galaxy client every time.

Various tests produced the following error:
```
Set $GALAXY_VIRTUAL_ENV to /home/travis/.planemo/gx_venv
/bin/sh: 1: if [ -e "$GALAXY_VIRTUAL_ENV" ]; : not found
run command exited with return code 127
```

Introduced in commit 434099e .

Fix wrong tool version in workflow.

Otherwise the `pid_file` is set to the wrong filename, preventing
`LocalGalaxyConfig.kill()` from working when running Galaxy under Python 3.

Follow-up to galaxyproject#874 .

Fix the following traceback:

```
Problem serving Galaxy
Traceback (most recent call last):
  File "/home/travis/build/galaxyproject/planemo/planemo/galaxy/serve.py", line 22, in serve
    return _serve(ctx, runnables, **kwds)
  File "/home/travis/build/galaxyproject/planemo/planemo/galaxy/serve.py", line 43, in _serve
    cmd = config.startup_command(ctx, **kwds)
  File "/home/travis/build/galaxyproject/planemo/planemo/galaxy/config.py", line 963, in startup_command
    host=kwds['host'],
KeyError: 'host'
```