Allow customized MSZ Option 57 instead of hard-coded 1500

[w13915984028]

The dhcp v4 client will set the MSZ Option 57 as 1500 always, instead, most clients use the default minimal value 576

dhcp/dhcpv4/nclient4/client.go

Line 38 in ddd8a41

MaxMessageSize = 1500

e.g.: A dhcp client from here:

16:13:25.070626 52:54:00:e1:2e:23 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 342: (tos 0x10, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: [udp sum ok] BOOTP/DHCP, Request from 52:54:00:e1:2e:23, length 300, xid 0x49cc3e76, Flags [none] (0x0000)
	  Client-Ethernet-Address 52:54:00:e1:2e:23
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x63825363
	    DHCP-Message Option 53, length 1: Discover
	    Client-ID Option 61, length 19: hardware-type 255, 00:e1:2e:23:00:01:00:01:2e:2b:a9:b6:52:54:00:e1:2e:23
	    Parameter-Request Option 55, length 20: 
	      RN, RB, Subnet-Mask, BR
	      MTU, Classless-Static-Route, Default-Gateway, Static-Route
	      Option 119, Domain-Name, Domain-Name-Server, YD
	      YS, NTP, RP, Option 85
	      Option 86, Option 87, POSIX-TZ, TZ-Name
	    MSZ Option 57, length 2: 1500
	    Hostname Option 12, length 5: "harv2"

The ubuntu VM dhcp client:

    0.0.0.0.68 > 255.255.255.255.67: [udp sum ok] BOOTP/DHCP, Request from 52:54:00:ed:f1:a7, length 288, xid 0x3d01be60, secs 1, Flags [none] (0x0000)
	  Client-Ethernet-Address 52:54:00:ed:f1:a7
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x63825363
	    DHCP-Message Option 53, length 1: Discover
	    Client-ID Option 61, length 19: hardware-type 255, 56:50:4d:98:00:02:00:00:ab:11:d2:7c:59:26:b0:aa:ac:77
	    Parameter-Request Option 55, length 11: 
	      Subnet-Mask, Default-Gateway, Hostname, Domain-Name
	      Domain-Name-Server, MTU, Static-Route, Classless-Static-Route
	      Option 119, NTP, Option 120
	    MSZ Option 57, length 2: 576
	    Hostname Option 12, length 4: "wang"

In an issue, harvester/harvester#3428, we found the the first DHCPDiscover did not receive an response

The only notable difference is the Option 57 1500.

Could we allow the caller set this param, if set, the dhcp uses it?