added _try_assigned_hostname & unittests; WIP
fixed tests & reverted unique prop. for hostname (to rethink it..)
review changes
new concept save hostname by signals; added functional test for hostname
added tests test_hostname_is_eventually_assigned & fixed bug
fixed unittests, better exception during hostname assigning, flake8
added help_text to hostname
added lookup
added test for new lookup
added required statment
fixed AssetLookup
better tests
removed unique on contract_id
added migration
review improvements & small bug fix
simplified hostname field signal & rm useless tests
Merge pull request #344 from ar4s/324-non-unique-contract-id

Removed unique on contract_id
Merge branch 'develop' into hostname-use

Conflicts:
	src/ralph_assets/tests/functional/tests_view.py
review fixes
Merge branch 'develop' into 323-bo-with-supports