maxscale-docker: syntax error on rsyslog configuration

In dockerfile on this line:
sed -i 's|^.*module(load="imjournal"|#module(load="imjournal"|g' /etc/rsyslog.conf && \
the imjournal module is commented out. But it's contains other indented content like UsePid="system" which isn't included in this regex it results:

#module(load="imjournal"
UsePid="system"

1. StateFile="imjournal.state")

and errors like:
rsyslogd: error during parsing file /etc/rsyslog.conf, on or before line 13: warnings occured in file '/etc/rsyslog.conf' around line 13

we should modify the regex to comment all the configuration block, our instead use a predefined rsyslog.conf and copy in inside the docker image when build.

please let me know if you're agree on the approach and I'll create a PR.