This is breaking the MAAS CI at the moment as well. The CI does all network communication through the proxy. Here is the installation log:

Err http://archive.ubuntu.com trusty InRelease
  
Err http://archive.ubuntu.com trusty-updates InRelease
  
Err http://security.ubuntu.com trusty-security InRelease
  
Err http://archive.ubuntu.com trusty Release.gpg
  Unable to connect to archive.ubuntu.com:http:
Err http://security.ubuntu.com trusty-security Release.gpg
  Unable to connect to security.ubuntu.com:http:
Err http://archive.ubuntu.com trusty-updates Release.gpg
  Unable to connect to archive.ubuntu.com:http:
Reading package lists...
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/InRelease  

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-updates/InRelease  

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/trusty-security/InRelease  

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/Release.gpg  Unable to connect to archive.ubuntu.com:http:

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-updates/Release.gpg  Unable to connect to archive.ubuntu.com:http:

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/trusty-security/Release.gpg  Unable to connect to security.ubuntu.com:http:

W: Some index files failed to download. They have been ignored, or old ones used instead.
Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package bcache-tools
Unexpected error while running command.
Command: ['eatmydata', 'apt-get', 'install', '--quiet', '--assume-yes', '--option=Dpkg::options::=--force-unsafe-io', 'mdadm', 'lvm2', 'bcache-tools']
Exit code: 100
Reason: -
Stdout: ''
Stderr: ''
Installation failed with exception: Unexpected error while running command.
Command: ['curtin', 'block-meta', 'custom']
Exit code: 3
Reason: -
Stdout: "Err http://archive.ubuntu.com trusty InRelease\n  \nErr http://archive.ubuntu.com trusty-updates InRelease\n  \nErr http://security.ubuntu.com trusty-security InRelease\n  \nErr http://archive.ubuntu.com trusty Release.gpg\n  Unable to connect to archive.ubuntu.com:http:\nErr http://security.ubuntu.com trusty-security Release.gpg\n  Unable to connect to security.ubuntu.com:http:\nErr http://archive.ubuntu.com trusty-updates Release.gpg\n  Unable to connect to archive.ubuntu.com:http:\nReading package lists...\nW: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/InRelease  \n\nW: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-updates/InRelease  \n\nW: Failed to fetch http://security.ubuntu.com/ubuntu/dists/trusty-security/InRelease  \n\nW: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/Release.gpg  Unable to connect to archive.ubuntu.com:http:\n\nW: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-updates/Release.gpg  Unable to connect to archive.ubuntu.com:http:\n\nW: Failed to fetch http://security.ubuntu.com/ubuntu/dists/trusty-security/Release.gpg  Unable to connect to security.ubuntu.com:http:\n\nW: Some index files failed to download. They have been ignored, or old ones used instead.\nReading package lists...\nBuilding dependency tree...\nReading state information...\nE: Unable to locate package bcache-tools\nUnexpected error while running command.\nCommand: ['eatmydata', 'apt-get', 'install', '--quiet', '--assume-yes', '--option=Dpkg::options::=--force-unsafe-io', 'mdadm', 'lvm2', 'bcache-tools']\nExit code: 100\nReason: -\nStdout: ''\nStderr: ''\n"
Stderr: ''