"home" plug doesn't work with encrypted home dir

[Casey Marshall]

$ snap --version
snap 2.0.10
snapd 2.0.10
series 16
ubuntu 16.04

Steps to reproduce:

1. snap install hello-world
2.

$ /snap/bin/hello-world
failed to create user data directory. errmsg: Permission denied

/var/log/kern.log:

Jul 29 09:34:17 mawhrin-skel kernel: [ 5247.286681] audit: type=1400 audit(1469777657.669:50): apparmor="DENIED" operation="open" profile="/usr/bin/ubuntu-core-launcher" name="/home/.ecryptfs/c/.Private/" pid=11244 comm="ubuntu-core-lau" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
Jul 29 09:34:17 mawhrin-skel kernel: [ 5247.286689] ecryptfs_dir_open: Error attempting to initialize the lower file for the dentry with name [/]; rc = [-13]