deploy-ironic ramdisks do not boot: Failed to execute /init (error -28)

[Adam Gandelman]

The default ubuntu release for ramdisks was bumped to Trusty https://review.openstack.org/#/c/88384/ Ironic + devstack deployments stopped functioning. The ramdisk fails to run init, crashing with the following error:

[ 23.154016] Failed to execute /init (error -28)
[ 23.154909] Starting init: /sbin/init exists but couldn't execute it (error -28)
[ 23.156388] Starting init: /bin/init exists but couldn't execute it (error -28)
[ 23.157490] Starting init: /bin/sh exists but couldn't execute it (error -28)
[ 23.160313] Kernel panic - not syncing: No working init found. Try passing init= option to kernel. See Linux Documentation/init.txt for guidance.
[ 23.164146] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.13.0-24-generic #46-Ubuntu
[ 23.164146] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 23.164146] 0000000000000000 ffff88001a3a9ec0 ffffffff81715a64 ffffffff81a39f38
[ 23.164146] ffff88001a3a9f38 ffffffff8170ec65 0000000000000008 ffff88001a3a9f48
[ 23.164146] ffff88001a3a9ee8 ffffffff81a39a38 0000000013981396 0000000000001396
[ 23.164146] Call Trace:
[ 23.164146] [<ffffffff81715a64>] dump_stack+0x45/0x56
[ 23.164146] [<ffffffff8170ec65>] panic+0xc8/0x1d7
[ 23.164146] [<ffffffff81703f40>] ? rest_init+0x80/0x80
[ 23.164146] [<ffffffff81704062>] kernel_init+0x122/0x130
[ 23.164146] [<ffffffff8172637c>] ret_from_fork+0x7c/0xb0
[ 23.164146] [<ffffffff81703f40>] ? rest_init+0x80/0x80
[ 23.164146] general protection fault: fff2 [#1] SMP
[ 23.164146] Modules linked in:
[ 23.164146] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.13.0-24-generic #46-Ubuntu
[ 23.164146] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 23.164146] task: ffff88001a3a0000 ti: ffff88001a3a8000 task.ti: ffff88001a3a8000
[ 23.164146] RIP: 0010:[<ffffffff8170ed30>] [<ffffffff8170ed30>] panic+0x193/0x1d7
[ 23.164146] RSP: 0000:ffff88001a3a9ed0 EFLAGS: 00000246
[ 23.164146] RAX: 0000000014a814a8 RBX: ffffffff81a39f38 RCX: 0000000000000854
[ 23.164146] RDX: 00000000000014a8 RSI: 0000000000000000 RDI: 0000000000000046

#define ENOSPC 28 /* No space left on device */