Get that journalctl log showing networkmanager starting before the related kernel'module been ready:
systemd[1]: Starting Network.
systemd[1]: Starting /etc/rc.local Compatibility...
NetworkManager[724]: nm_device_get_device_type: assertion 'NM_IS_DEVICE (self)' failed
NetworkManager[724]: <info> (lo): link connected
NetworkManager[724]: <info> (lo): carrier is ON
NetworkManager[724]: <info> (lo): new Generic device (driver: 'unknown' ifindex: 1)
NetworkManager[724]: <info> (lo): exported as /org/freedesktop/NetworkManager/Devices/0
NetworkManager[724]: nm_device_get_device_type: assertion 'NM_IS_DEVICE (self)' failed
NetworkManager[724]: <info> (eth0): carrier is OFF
NetworkManager[724]: <info> (eth0): new Ethernet device (driver: 'sky2' ifindex: 2)
NetworkManager[724]: <info> (eth0): exported as /org/freedesktop/NetworkManager/Devices/1
NetworkManager[724]: <info> (eth0): device state change: unmanaged -> unavailable (reason 'managed') [10 20 2]
NetworkManager[724]: <info> (eth0): preparing device
NetworkManager[724]: nm_device_get_device_type: assertion 'NM_IS_DEVICE (self)' failed
NetworkManager[724]: <info> (eth1): carrier is OFF
NetworkManager[724]: <info> (eth1): new Ethernet device (driver: 'sky2' ifindex: 3)
NetworkManager[724]: <info> (eth1): exported as /org/freedesktop/NetworkManager/Devices/2
NetworkManager[724]: <info> (eth1): device state change: unmanaged -> unavailable (reason 'managed') [10 20 2]
kernel: sky2 0000:04:00.0 eth0: enabling interface
kernel: sky2 0000:03:00.0 eth1: enabling interface
NetworkManager[724]: <info> (eth1): preparing device
NetworkManager[724]: <info> urfkill disappeared from the bus
that mobo have eth0 & eth1 ports, but only eth1 is used. As the last lines logged, networworkmanager is able to be loaded when the module sky2 is enabled.
network-manager 0.9.10.0-4ubuntu13
ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: systemd 219-6ubuntu3
ProcVersionSignature: Ubuntu 3.19.0-12.12-generic 3.19.3
Uname: Linux 3.19.0-12-generic i686
NonfreeKernelModules: nvidia
ApportVersion: 2.17-0ubuntu1
Architecture: i386
CurrentDesktop: GNOME
Date: Thu Apr 9 11:41:02 2015
Lsusb:
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 002: ID 046d:c50e Logitech, Inc. Cordless Mouse Receiver
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
MachineType: ASUSTEK COMPUTER INC P5W DH Deluxe
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.19.0-12-generic root=UUID=7b1f4a51-558f-468f-85e0-f815d2f791e1 ro
SourcePackage: systemd
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 07/22/2010
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 3002
dmi.board.asset.tag: To Be Filled By O.E.M.
dmi.board.name: P5W DH Deluxe
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: Asset-1234567890
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr3002:bd07/22/2010:svnASUSTEKCOMPUTERINC:pnP5WDHDeluxe:pvrSystemVersion:rvnASUSTeKComputerINC.:rnP5WDHDeluxe:rvrRev1.xx:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.name: P5W DH Deluxe
dmi.product.version: System Version
dmi.sys.vendor: ASUSTEK COMPUTER INC
Network devices come and go, some are hotpluggable, some have killswitches, etc. There is no such concept as "wait for all network devices to be present". NetworkManager listens to uevents and can handle appearing and disappearing network interfaces just fine.
If that doesn't work for a particular device of your's, can you please file a bug against network-manager, with specific details about the bug you are seeing? thanks!