rename password file to consistent name

When overlayroot is used with config of:
  overlayroot=crypt:dev=/dev/vdc,debug=1,mkfs=1

Then the password that is used for the mount is recorded to
a file in in /run/initramfs (or /dev/.initramfs on older initramfs-tools).
The file name is currently:
   overlayroot.XXXXXX
where the 'XXXXXX' is random as it is the result of
   mktemp overlayroot.XXXXXX

There doesn't seem to be any value in having that filename be random.
Some things to note:
 a.) file is on a tmpfs (initially created in the initramfs and then moved with the pivot)
 b.) file does (and should) have root:root permissions and 0600

It seems a general better idea to make that /run/initramfs/overlayroot.passwd

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: overlayroot 0.31ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-45.66-generic 4.4.21
Uname: Linux 4.4.0-45-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.3-0ubuntu8
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Nov 14 12:28:29 2016
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-07-23 (480 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150722.1)
PackageArchitecture: all
SourcePackage: cloud-initramfs-tools
UpgradeStatus: No upgrade log present (probably fresh install)