Sometimes it happens that st2 group or mistral group is not created with packages.
Inexistent group results that all services fail to start (except of st2actionrunner which is launched from the root:root).

So both:

needs better control.

Clean machine (working):

sudo adduser --no-create-home --system st2test1
$ groups st2test1
st2test1 : st2test1

Here is a disconnect on affected machine:

sudo adduser --no-create-home --system st2test1
$ groups st2test1
st2test1 : users

(probably because of custom /etc/default/useradd config)

Also TODO: Add additional tests where possible.