I had this issue too behind a proxy:

sudo su -

export http_proxy=http://PROXY-IP:80/

export https_proxy=http://PROXY-IP:80/

sed -i 's/"keyserver.ubuntu.com"/"hkp:\/\/keyserver.ubuntu.com:80"/' /usr/share/pyshared/softwareproperties/ppa.py

add-apt-repository ppa:nova-core/release

did the trick for me