Validate brute-force combinations

**Is your feature request related to a problem? Please describe.**
Agents capabilities as brute forcer are limited. We need to figure out and express this limitation in configuration schema.

**Describe the solution you'd like**
We should validate that the user didn't add too many brute-force combinations.

## Tasks
- [ ] Figure out what's a reasonable number of brute force attempts. This will require to take a look at how long it takes for a typical brute force attempt to fail (ssh/wmi). Also, take into consideration security solutions like sshguard (enabled by default in Ubuntu), which increase throttling with every failed request.
- [ ] Add validation rules to the schema to make sure that in total `usernames * passwords` or `usernames * ssh_keys` or `usernames * ntlm_hashes` doesn't go over a certain number. If it does, show an error message, that the maximum number of credentials have been exceeded.