Default was previously NOTICE (as in repmgr 3.x) but documentation
implied it was INFO, and many of the the documentation examples assume
it is.
This produces some quite informative log output, without creating excessive
log file volume. In particular it's useful to get a better idea of what
repmgrd is actually doing.
Also add documentation section for the log configuration parameters.
GitHub #470, containing change suggested in GitHub #467.
