opm-core installation & troubleshooting doc updates

[Krysztophe]

Documentation updates:

• a few typos and formulations, a bit of formatting
• some more details that I think would be important for someone installing OPM without knowing much about nagios, PG extensions, OPM itself, like distribution packages
• a few traps I fell in :-)
• changed "user1" name and the identical similar password
• changed daemon=0 in the configuration file for systemd (it does not work with =1 on my setup - btw, does it with inittab or upstart ??) ;
• in same file, I've kept only the lines different from above
• added a warning that debug=1 stops the process when there is a problem (that's unexpected)

[rjuju]

Thanks for the PR, it's a lot of good enhancements! I requested some changes (with some unrelated to your changes).

[rjuju]

Merged, thanks!