Description of problem:

Version-Release number of selected component (if applicable):
Satellite 6.15.0 Snap 4.0

How reproducible:
Always

Steps to Reproduce:
1. # hammer settings set --id discovery_prefix --value '$@%#*&'
Setting [discovery_prefix] updated to [$@%#*&].

2. # hammer settings info --name discovery_prefix
Id: discovery_prefix
Name: discovery_prefix
Description: The default prefix to use for the host name, must start with a letter
Category: Discovery
Settings type: string
Value: $@%#*&

Actual results:
Invalid values like '$@%#*&', '1mac', '-mac', etc can be set to setting discovery_prefix

Expected results:
Invalid values shouldn't be set to setting discovery_prefix, as per description "The default prefix to use for the host name, must start with a letter"

Additional info:
Previously, this setting had validation added when it could not be set to null, https://bugzilla.redhat.com/show_bug.cgi?id=1911228