Fix e2e mail test

The e2e test timeouts after:

$ make test_e2e
# if there's no sudo, assumming this is running as root by the CI
test -f /usr/bin/sudo && sudo cp src/leap/bitmask/vpn/helpers/linux/bitmask-root /usr/local/sbin/ || cp src/leap/bitmask/vpn/helpers/linux/bitmask-root /usr/local/sbin/
test -f /usr/bin/sudo && sudo cp src/leap/bitmask/vpn/helpers/linux/se.leap.bitmask.policy /usr/share/polkit-1/actions/se.bitmask.bundle.policy || cp src/leap/bitmask/vpn/helpers/linux/se.leap.bitmask.policy /usr/share/polkit-1/actions/se.bitmask.bundle.policy
tests/e2e/e2e-test-mail.sh
Running with xtrace disabled!
WARNING:vext:Could not add extra path: /usr/share/sip
stop      ok
WARNING:vext:Could not add extra path: /usr/share/sip
start     ok
WARNING:vext:Could not add extra path: /usr/share/sip
signup    ok
user      tmp_user_20171123191811
created user. authenticating...
WARNING:vext:Could not add extra path: /usr/share/sip
WARNING:vext:Could not add extra path: /usr/share/sip
WARNING:vext:Could not add extra path: /usr/share/sip
…

The upgrade to the artful docker image seems related, because before the e2e test was working fine.

When running in verbose mode, tests/e2e/e2e-test-mail.sh loops because bitmaskctl mail get_token doesn't get a token back:

(py27-dev) root@runner--project-0-concurrent-0:/builds/project-0# bash -x tests/e2e/e2e-test-mail.sh
++ echo 'token     None
user      tmp_user_20171124085521@mail.bitmask.net'
++ head -n 1
++ sed 's/  */ /g'
++ cut '-d ' -f 2
+ imap_pw=None
+ [[ None == *\N\o\n\e* ]]
++ bitmaskctl mail get_token
+ response='token     None
user      tmp_user_20171124085521@mail.bitmask.net'
+ sleep 2