Disable ONLY_FULL_GROUP_BY Mysql mode when creating connection

[tsteur]

refs #5124

Workaround for Piwik not being compatible with mysql mode ONLY_FULL_GROUP_BY. Idea is to disable it until we have a proper solution (being compatible) for it.

I'm not sure re the SQL modes that we set but was recommended by a user in #5124 (comment) and sounds reasonable. We could make it configurable but would prefer to do this only when needed.

[diosmosis]

Will a proper solution be implemented for 3.0? If so, perhaps #5124 should be put into the 3.0 milestone.

CC @mattab

[diosmosis]

Instead of setting the whole SQL mode, would it be a better solution to replace the existing SQL mode via a method like http://johnemb.blogspot.com/2014/09/adding-or-removing-individual-sql-modes.html ?

[tsteur]

It would be a bit slower to do this on each request and I'd prefer to set the same SQL mode on each instance unless there's a problem. This way we - hopefully - won't have any weird bug reports that are hard to debug and caused by a different SQL mode. It should make our life easier and more predictable.

I don't think that it will be implemented for 3.0 but rather somewhen later.

[diosmosis]

I think this would likely result in less bug reports anyway, so I think this is a good idea for LTS. In fact, maybe enforcing as strict an SQL mode as possible would be a good idea (in the long run)? Anyway, will merge this PR.

[diosmosis]

Ok, there are some system test failures; I restarted the build, if it passes I'll merge.

[diosmosis]

Test failures appear to be in master, rebased myself.