Revise logging usage in openquake

[Anton Gritsay]

1. Write a wiki page with a clear explanation of logging level and names meanings and guidelines of using them.

2. Make logging in openquake project follow that guidelines.

[beatpanic (kpanic)]

Regarding info/debug/warn levels and logging performance

16:05 < angri> beatpanic: tested with deterministic_event_based_risk: takes 2
               min 56 sec on master on my machine and 3 min 2 sec on logging
               branch
16:05 < angri> even with --debug=debug
16:07 < angri> but it may be much slower for some other tests. simplecase for
               example produces crazy flood of log messages from java and from
               serializer so it is almost two times slower with debug enabled
16:09 < angri> but when there are not tens thousands of log messages or they
               are suppressed by --debug=warn the performance difference is
               neglectable
16:11 < beatpanic> angri, okay, should we maybe change some meaningful messages in
                   'info' logging level instead of debug? because IIRC there
                   are messages useful for the user like
                   https://bugs.launchpad.net/openquake/+bug/838984 but they
                   could be logged as 'info' I guess

our users are used to run --debug=debug on command line to have informational messages, given the "discussion" on the channel we could convert the most important 'debug' messages to less 'loud log level', like 'info' to avoid logging messages pollution and performance problems

[beatpanic (kpanic)]

we also have to inform our users that --debug=debug is really a development option, not a log level to use for production

[John Tarter (toh2)]

Ben, I thought that this fits nicely with the documentation that is being created on the wiki.