Plugin requires IP instead of hostname

[stdevel]

When specifying a hostname instead of an IP, the plugin crashes:

$ ./check_dlink_dgs.py -H myswitch.test.loc -c giertz -A
Traceback (most recent call last):
  File "./check_dlink_dgs.py", line 257, in <module>
    elif options.act_ports: get_active_ports()
  File "./check_dlink_dgs.py", line 136, in get_active_ports
    get_all_ports()
  File "./check_dlink_dgs.py", line 119, in get_all_ports
    check_switch_ports(True)
  File "./check_dlink_dgs.py", line 167, in check_switch_ports
    get_oid_result(OID)
  File "./check_dlink_dgs.py", line 99, in get_oid_result
    retryCount=5
  File "/usr/lib/python2.7/site-packages/pysnmp/entity/config.py", line 283, in addTargetAddr
    transportAddress = SnmpUDPAddress(transportAddress)
  File "/usr/lib/python2.7/site-packages/pyasn1/type/univ.py", line 284, in __init__
    base.AbstractSimpleAsn1Item.__init__(self, value, tagSet, subtypeSpec)
  File "/usr/lib/python2.7/site-packages/pyasn1/type/base.py", line 68, in __init__
    value = self.prettyIn(value)
  File "/usr/lib/python2.7/site-packages/pysnmp/smi/mibs/SNMPv2-TM.py", line 27, in prettyIn
    int2oct((value[1] >> 8) & 0xff) + \
socket.error: illegal IP address string passed to inet_pton