Hostname rename to host

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no (does not exist in 2.1)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6775
| License       | MIT
| Doc PR        | symfony/symfony-docs#2165

see #6775