Manual addition of write permission for user required for /var/lib/dhcp/dhcpd.leases

Without clear reason `isc-dhcp-server`'s `systemd` unit fails with error

    $ sudo systemctl status -l isc-dhcp-server
    ● isc-dhcp-server.service - ISC DHCP IPv4 server
       Loaded: loaded (/lib/systemd/system/isc-dhcp-server.service; enabled; vendor preset: enabled)
       Active: failed (Result: exit-code) since Do 2015-10-01 23:48:16 CEST; 1s ago
         Docs: man:dhcpd(8)
      Process: 31328 ExecStart=/bin/sh -ec CONFIG_FILE=/etc/dhcp/dhcpd.conf; if [ -f /etc/ltsp/dhcpd.conf ]; then CONFIG_FILE=/etc/ltsp/dhcpd.conf; fi; [ -e /var/lib/dhcp/dhcpd.leases ] || touch /var/lib/dhcp/dhcpd.leases; chown root:root /var/lib/dhcp /var/lib/dhcp/dhcpd.leases*; exec dhcpd -user dhcpd -group dhcpd -f -4 -pf /run/dhcp-server/dhcpd.pid -cf $CONFIG_FILE $INTERFACES (code=exited, status=1/FAILURE)
      Process: 31326 ExecStartPre=/bin/chown dhcpd:dhcpd /run/dhcp-server (code=exited, status=0/SUCCESS)
     Main PID: 31328 (code=exited, status=1/FAILURE)

    Okt 01 23:48:16 richtercloud systemd[1]: isc-dhcp-server.service: main process exited, code=exited, status=1/FAILURE
    Okt 01 23:48:16 richtercloud sh[31328]: Can't open /var/lib/dhcp/dhcpd.leases for append.
    Okt 01 23:48:16 richtercloud sh[31328]: If you think you have received this message due to a bug rather
    Okt 01 23:48:16 richtercloud sh[31328]: than a configuration issue please read the section on submitting
    Okt 01 23:48:16 richtercloud sh[31328]: bugs on either our web page at www.isc.org or in the README file
    Okt 01 23:48:16 richtercloud sh[31328]: before submitting a bug. These pages explain the proper
    Okt 01 23:48:16 richtercloud sh[31328]: process and the information we find helpful for debugging..
    Okt 01 23:48:16 richtercloud sh[31328]: exiting.
    Okt 01 23:48:16 richtercloud systemd[1]: Unit isc-dhcp-server.service entered failed state.
    Okt 01 23:48:16 richtercloud systemd[1]: isc-dhcp-server.service failed.

`$ ls -la /var/lib/dhcp/dhcpd.leases` indicates that write permissions are missing

    -rw-r--r-- 2 root root 49783 Aug 27 20:32 /var/lib/dhcp/dhcpd.leases

Update/`dpkg` script should assert that permission are set correctly and request action from the user in case of doubt.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: isc-dhcp-server 4.3.1-5ubuntu2.2
ProcVersionSignature: Ubuntu 3.19.0-30.33-generic 3.19.8-ckt6
Uname: Linux 3.19.0-30-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.17.2-0ubuntu1.5
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Oct 1 23:51:05 2015
DhServerLeases:

InstallationDate: Installed on 2015-04-20 (164 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
SourcePackage: isc-dhcp
UpgradeStatus: Upgraded to vivid on 2015-04-24 (160 days ago)
mtime.conffile..etc.dhcp.dhcpd.conf: 2015-08-31T13:31:13.948310