With disk encryption enabled, partition table will be changed right after security key setup

[Po-Hsu Lin]

Steps:
1. Install any other version of Ubuntu.
2. Boot with 16.04.1 dailylive (0208) LiveUSB
3. Select "Erase Disk and install Ubuntu", enable encryption and LVM
4. Setup your security key, and click "Install Now"

Expected results:
* Change should be made only after the "Write the change to disks?" dialogue, I can still change my mind to cancel the installation and reboot to my old system.

Actual results:
* Once you click on the "Install Now" button, changes will be made immediately to the disk, you won't be able to boot to your old system. The "Write the change to disks?" dialogue will appears later.

Another scenario is, if I want to change my security key after clicking on the "Install Now" button, I can do this by clicking on the "Cancel" button of the upcoming "Write the change to disks?" dialogue. However, as the change has already been made to the disk, it will complains about "Configuration failed" this time, stop user from completing the installation.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubiquity 2.21.63.3
ProcVersionSignature: Ubuntu 4.4.0-63.84-generic 4.4.44
Uname: Linux 4.4.0-63-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CasperVersion: 1.376.2
CurrentDesktop: Unity
Date: Thu Feb 9 15:22:29 2017
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash ---
LiveMediaBuild: Ubuntu 16.04.1 LTS "Xenial Xerus" - Beta amd64 (20170208)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Ubuntu QA Website (ubuntuqa)]

This bug has been reported on the Ubuntu ISO testing tracker.

A list of all reports related to this bug can be found here:
http://iso.qa.ubuntu.com/qatracker/reports/bugs/1663143