1.25 ppa incompatible with xenial

[Ryan Beisner]

The 1.25 ppa is incompatible with Xenial distro packaging, as it replaces the /usr/bin/juju symlink with a 1.25 binary file.

This makes it extremely difficult to switch between juju 2 and juju 1 on a Xenial system, such as we must do in OpenStack Charm CI.

Typically in Xenial, the /usr/bin/juju symlink can be pointed at either /usr/bin/juju-2.0 or /usr/bin/juju-1.25 to switch between default `juju` command versions.

Also note that when upgrading from Xenial juju-1.25 (1.25.6) to 1.25.8 from the ppa, a mix of 1.25.6 and 1.25.8 package and binary versions exist on the machine:

ii juju 1:2.0.0-0ubuntu1~16.04.2~juju1 all next generation service orchestration system
ii juju-1.25 1.25.6-0ubuntu1.16.04.1 amd64 Juju is devops distilled - client
ii juju-2.0 1:2.0.0-0ubuntu1~16.04.2~juju1 amd64 Juju is devops distilled - client
ii juju-core 1.25.8-0ubuntu1~16.04.1~juju1 amd64 Juju is devops distilled - client

[Ryan Beisner (1chb1n)]

For OSCI infra, we'll need to implement some post-installation glue to move binaries and reconstruct the expected symlink:binaries reality we've come to expect on Xenial.

[Ryan Beisner (1chb1n)]

After removing all juju packages, then enabling the 2 and 1 ppas, and re-installing:

ubuntu@juju-osci1-machine-8:~$ juju version
The program 'juju-1' is currently not installed. You can install it by typing:
sudo apt install juju-1.25

ubuntu@juju-osci1-machine-8:~$ which juju
/usr/bin/juju

[Ryan Beisner (1chb1n)]

We've successfully but temporarily worked around this with some post-installations mv and ln -sf foo.