api_test.py in ubuntu_lxc failed with "ERROR: Invalid signature" on T-lts

Issue found on 4.4.0-214.246~14.04.1 with node "kili" generic kernel.

Test failed with:
 Running 'python3 /usr/share/doc/python3-lxc/examples/api_test.py'
 Setting up the GPG keyring
 Downloading the image index
 ERROR: Invalid signature for /tmp/tmp.OzFQ8feRBf/index.asc
 lxc_container: lxccontainer.c: create_run_template: 1092 container creation template for 8dd0d06a-0aeb-11ec-984b-001e67d810ba failed
 Getting instance for '8dd0d06a-0aeb-11ec-984b-001e67d810ba'
 Creating rootfs using 'download', arch=amd64
 Traceback (most recent call last):
   File "/usr/share/doc/python3-lxc/examples/api_test.py", line 72, in <module>
     assert(container.defined)
 AssertionError

This should not be consider as a regression since we have change our test case in ubuntu_lxc recently (bug 1939859), this api_test.py was failing with GPG key fetch in bug 1916087

Note that this issue does not happen on T-lts lowlatency, which failed with "ERROR: Couldn't find a matching image." instead