kea-dhcp4 server needs to support configurable client hostname sanitization

The Infoblox SOW requires the ability to sanitize client host name values, sent by DHCPv4 clients, prior to using them to form the FQDN for DNS updates. Specifically, there should be two configuration parameters:

1. A regular expression (e.g. [A-za-z0-9]-) which describes the allowed characters
2. A replacement for characters that are disallowed (e.g. _ by -)

Does not apply to v6, or to v4 FQDN option values, as per RFC they cannot contain non-compliant characters.