Ubiquity fails to install grub when /boot is encrypted

[Andrew Gunnerson]

Existing partition setup:
- /dev/sda1 - EFI
- /dev/sda2 - LUKS encrypted volume for LVM PV

Within encrypted LVM PV:
- vg/ubuntu-root - Root volume for Ubuntu (already exists)
- vg/ubuntu-home - Home volume for Ubuntu (already exists)

Partition options selected during installation:
- /dev/sda1 - Use as EFI partition
- /dev/vg/ubuntu-root - Format as ext4, mount at /
- /dev/vg/ubuntu-home - Format as ext4, mount at /home

For Ubiquity to detect the LVM volumes at all, I first had to open the encrypted partition manually with: `sudo cryptsetup open /dev/sda2 crypt`. After that, the installation proceeded as normal until it got to the grub installation step and failed. It seems to be because /boot is encrypted in my setup and that requires `GRUB_ENABLE_CRYPTODISK=y` to be set in `/etc/default/grub`.

```
/var/log/syslog:Oct 21 17:19:42 ubuntu ubiquity: Setting up grub-efi-amd64-signed (1.85+2.02~beta3-4ubuntu7) ...
/var/log/syslog:Oct 21 17:19:48 ubuntu ubiquity: File descriptor 4 (/dev/sda1) leaked on vgs invocation. Parent PID 16464: grub-install
/var/log/syslog:Oct 21 17:19:48 ubuntu ubiquity: File descriptor 4 (/dev/sda1) leaked on vgs invocation. Parent PID 16464: grub-install
/var/log/syslog:Oct 21 17:19:48 ubuntu ubiquity: grub-install: error: attempt to install to encrypted disk without cryptodisk enabled. Set `GRUB_ENABLE_CRYPTODISK=y' in file `/etc/default/grub'.
/var/log/syslog:Oct 21 17:19:48 ubuntu ubiquity: dpkg: error processing package grub-efi-amd64-signed (--configure):
```

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: ubiquity 17.10.10
ProcVersionSignature: Ubuntu 4.13.0-16.19-generic 4.13.4
Uname: Linux 4.13.0-16-generic x86_64
ApportVersion: 2.20.7-0ubuntu3
Architecture: amd64
CasperVersion: 1.387
Date: Sat Oct 21 13:24:50 2017
LiveMediaBuild: Ubuntu 17.10 "Artful Aardvark" - Release amd64 (20171018)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Ubuntu Foundations Team Bug Bot (crichton)]

Exception during installation:
Oct 21 17:20:56 ubuntu /plugininstall.py: Traceback (most recent call last):
Oct 21 17:20:56 ubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 1721, in <module>
Oct 21 17:20:56 ubuntu /plugininstall.py: install.run()
Oct 21 17:20:56 ubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 61, in wrapper
Oct 21 17:20:56 ubuntu /plugininstall.py: func(self)
Oct 21 17:20:56 ubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 227, in run
Oct 21 17:20:56 ubuntu /plugininstall.py: self.configure_bootloader()
Oct 21 17:20:56 ubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 971, in configure_bootloader
Oct 21 17:20:56 ubuntu /plugininstall.py: "GrubInstaller failed with code %d" % ret)
Oct 21 17:20:56 ubuntu /plugininstall.py: ubiquity.install_misc.InstallStepError: GrubInstaller failed with code 1
Oct 21 17:20:56 ubuntu /plugininstall.py: