kea-admin lease-* options to be renamed to db-*

for now kea-admin options looks like that:

wlodek@debian9-64-2:sbin $ ./kea-admin      
ERROR/kea-admin: missing command
kea-admin 1.5.0-git

This is a kea-admin script that conducts administrative tasks on
the Kea installation.

Usage: ./kea-admin COMMAND BACKEND [parameters]

COMMAND: Currently supported operations are:

 - lease-init: Initializes new lease database. Useful for first time installation.
 - lease-version: Checks version of the existing lease database scheme. Useful
 -                for checking lease DB version when preparing for an upgrade.
 - lease-upgrade: Upgrades your lease database scheme
 - lease-dump: Dump current leases to a CSV file

But now we use one db schema for leases and config. I propose to update commands lease-init, lease-version, lease-upgrade, lease-dump for more suitable names. Having said that - I don't have propositions for new names.