Better handling of default sections

[rhertzog]

This pull request adds a way to disable the default sections defined in samba/defaults.yaml and it drops the default "files" section which seems to be a misplaced example entry. "homes" and "printers" sections are real default sections however.

[mstarostik]

Just got bitten by this today as well. In fact I don't think any share section should be in the defaults. At least the servers I usually deal with don't export either homes nor printers. These should just be examples, not defaults IMHO.

[noelmcloughlin]

LTGM. thx