jenkins user can't run commands from snaps

[dannf]

The scalebot-jenkins charm is layered on top of the jenkins charm. It uses an internal juju controller to deploy tests. Now that the jenkins charm supports bionic, we've been updating scalebot-jenkins to bionic as well. One change we need to make is to use the juju snap internally instead of the deb. This caused things to break:

ubuntu@ip-10-230-186-23:~$ sudo su - jenkins
jenkins@ip-10-230-186-23:~$ juju
cannot create user data directory: /var/lib/jenkins/snap/juju/6849: Not a directory

Turns out the root cause here is that /var/lib/jenkins is a symlink, see LP: #1620771 for details.

This could be fixed by having the charm use a bind mount instead of a symlink - manually making this change after deploy WFM:

ubuntu@ip-10-230-186-23:~$ sudo su - jenkins
jenkins@ip-10-230-186-23:~$ juju
Usage: juju [help] <command>

Summary:
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