Make server_address/command_timeout configurable, allow to use checks outside of the nrpe dir

This pull request contains three commits:

- Allow to specify the server address to avoid binding on all interfaces/IP addresses for security/whatever reasons
- Allow to change the command timeout, for slowish systems or slowish checks
- Allow to use commands outside of the nrpe plugin directory - for example to run a command with sudo or to use checks form other directories. Might not be the perfer approach, but works well for us.