Not configured control sockets - descriptive hint or config checker

Two users (#1041 (closed) and #1042 (closed)) reported the same misconfiguration issue with missing control socket entries in the DHCP daemon configurations last week. It caused the Kea Control Agent couldn't forward Strok commands to the DHCP daemons.

Let's do four things:

• Handle the problem on Stork UI and display a descriptive message with a suggested configuration block to add. Currently, there is a generic message about connection issues.
• Add a config review checker for this problem. I don't know if it is possible for the DHCP daemons because we cannot fetch the configuration without control sockets. But we can do it at least for Kea Control Agent.
• The users probably installed the Kea from the packages available in the operating system repository (Debian?). We should check the default configuration provided by these packages and contact the maintainers if the control socket section is missing by default to change it.
• Describe the problem and solution in the Stork troubleshooting docs.