Added new ServicesProposal library

- needed for bnc#887688
- Yast Network will have to set it (sshd)
- Yast Services Manager will have to read it and export it within its own configuration
- The test passes
- The package builds
YAGNI.

the return value is not used anywhere which means it's superfluous.  point in case: you managed to get it wrong (only in a comment but still) in the tests.

it would make more sense to me to return `self` to enable method call chaining.