provider/lxd: handle concurrent AddCert

[axw]

Description of change

Fix a TOCTOU bug, where we check if the
certificate exists on the server and then
add it if it's not there. We must make
a second check if the update fails, to
see if there was a concurrent add.

QA steps

How do we verify that the change works?

• Remove all trust certificates from lxd: lxc config trust list | grep '|' | grep -v FINGERPRINT | cut -d'|' -f2 | awk '{print $1}' | xargs -L1 lxc config trust remove
• for x in {1..10}; do juju bootstrap localhost lxd-$x & done

Documentation changes

None.

Bug reference

Fixes https://bugs.launchpad.net/juju/+bug/1661458

[axw]

$$fixes-1661458$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju