Check units are already suggested before suggesting them.

This patch adds a missing check for already suggested units.
This check causes units not to be repeated in same physical interface, even when the interface has no vlan.

This patch additionally creates following methods for not repeating code:
- markAsSuggestedUnit
- markAsSuggestedVlan
And calls them where required.
Merge branch 'develop' of git://github.com/dana-i2cat/opennaas into fix/units-suggestion-repeats-values
Merge pull request #494 from isartcanyameres/fix/units-suggestion-repeats-values

Fix/units suggestion repeats values