add test for create subnet with default gateway and conflict allocation pool

add test for create subnet with default gateway and conflict allocation pool

Create subnet with default gateway and conflict allocation pool should return
status code 409:
$ quantum subnet-create net1 --allocation-pool start=10.0.0.1,end=10.0.0.5 \
10.0.0.0/24
Found overlapping allocation pools:{u'start': u'10.0.0.1',
u'end': u'10.0.0.5'} 10.0.0.1 for subnet 10.0.0.0/24.

Change-Id: I8608314ad7899a752eea239dbe224e25c00a375b