OS-479, DHCP service workflow fixes

    * dhcp_members was asigned but infoblox services wasnt restarted
    * dns server wasnt assigned in infoblox DHCP service
It fills only here:

net.member_ip_addr = net.members[0]['ipv4addr']

But as i see we do not know what first member represent DHCP or DNS, so we may have next situation:

DNS member, opt['value'] = 1.1.1.1 // current DNS value
New DNS member, relay_ip = 2.2.2.2 // new DNS value
DHCP member, self.member_ip_addr = 3.3.3.3 // first member in list

So in this situation we will have:
"1.1.1.1".replace("3.3.3.3", "2.2.2.2")
And this line have no sence.

Will try to replace with somethig looks like:
opt['value'] = ",".join(list(set([relay_ip] + opt['value'].split(','))))

Looks a bit scary, but should work.