patch: avoid error if no dhcp plugin is present (FailedPythonPlugins: dhcp)

[sibelle-labs]

avoid error if no dhcp plugin is present from debug ...

line 583, in check_dhcp
    _dhcpconf = open("/var/dhcpd/etc/dhcpd.conf","r").read()
FileNotFoundError: [Errno 2] No such file or directory: '/var/dhcpd/etc/dhcpd.conf'
<<<check_mk>>>
FailedPythonPlugins: dhcp

patch:

<         if not os.path.exists("/var/dhcpd/var/db/dhcpd.leases"):
---
>         if not os.path.exists("/var/dhcpd/var/db/dhcpd.leases") or not os.path.exists("/var/dhcpd/etc/dhcpd.conf"):

[sibelle-labs]

now at pull request