Editing important global params

During the productization discussions, we came up with a need to be able to edit some of the more important global Kea parameters in Stork.

@dankney and @peterd provided this list:

• authoritative
• cache-threshold
• expired-lease-processing
• echo-client-id
• allocator
• early-global-reservations-lookup
• host-reservation-identifiers
• reservations-global
• reservations-in-subnet
• reservations-out-of-pool

And if you found yourself with an over abundance of time:

• ddns-send-updates
• ddns-override-no-update
• ddns-override-client-update
• ddns-replace-client-name
• ddns-generated-prefix
• ddns-qualifying-suffix
• ddns-update-on-renew
• ddns-use-conflict-resolution
• ddns-ttl-percent
• dhcp-ddns