Make host/interface/cname unique

- Host names are unique (no 2 hosts with same name)
- A host name shall not conflict with a cname or interface (if not main
interface)
- Interface names are unique (no 2 interfaces with same name)
- An interface shall not conflict with a cname or host (if not assigned
host)
- cnames are only unique by domain (2 cnames can have the same name if
different domains)
- A cname shall not conflict with a host or interface
- An interface has to be linked to a host

Validation is implemented at the model level.
Some is also implemented at the form level as it gives nicer feedback,
but not all as it's a bit more complex when it requires check on several fields.

JIRA INFRA-245