Allow alternative executables

With this commit it is possible to use virtualenv with the init scripts.

For example, I have this in my /etc/default/salt:

```
SALTMASTER=/srv/venvs/venv-1/bin/salt-master
SALTMINION=/srv/venvs/venv-1/bin/salt-minion
SALTSYNDIC=/srv/venvs/venv-1/bin/salt-syndic
SALTAPI=/srv/venvs/venv-1/bin/salt-api
PYTHON=/srv/venvs/venv-1/bin/python
```
Fix the LSB header

I got complaints from insserv that the LSB header was not compliant. It was
missing required keywords, which must be there even if they are empty.
Merge pull request #3704 from holmboe/alternative-salt-executables

Allow alternative executables