Refactoring bridge slave candidates list creation (InitBridge)

Separated UI and logic. Used relevant code from bonding.
Updated changelog.
Created testsuite for bridge.
Improved slave candidates list creation - exclude bonded devices.

For bridge device all bonded devices are excluded. For bond device
all devices bonded in another bond are excluded.
Updated testsuite.
Updated changelog.
Refactoring GetSlaveCandidates and relevant validators and helpers.

Added master device name as a parameter to make the function more
explicit.
Adopted testsuite.
Merge pull request #74 from mchf/Code-11-SP3

Refactoring candidates for enslaving list creation.