Make delete action on content_modifier processor accept list of keys

**Is your feature request related to a problem? Please describe.**
No.

**Describe the solution you'd like**
```
processors:
logs:
- name: content_modifier
action: delete
key:
- foo
- bar
- baz
```

**Describe alternatives you've considered**
N/A

**Additional context**
This would make the configuration a lot less bulky if one is removing a large amount of keys.