read_hosts_file_leases shouldn't parse stateless IPv6

read_hosts_file_leases shouldn't parse stateless IPv6

Error when _read_hosts_file_leases tries to parse stateless IPv6 entry
in hosts file
TRACE neutron.agent.dhcp.agent ip = host[2].strip('[]')
TRACE neutron.agent.dhcp.agent IndexError: list index out of range

Neutron creates entries in dhcp host file for each subnet of a port.
Each of these entries will have same mac address as first field,
and may have client_id, fqdn, ipv4/ipv6 address for dhcp/dhcpv6 stateful,
or tag as other fields.

For dhcpv6 stateless subnet with extra_dhcp_opts,
host file will have only mac address and tag. So _read_hosts_file_leases
shouldn't check for ip address for this entry in host file.

Closes-bug: #1465330
Change-Id: Iad6605ac5c7bcd6ec9204352037ed021f5007738