MySQL 5.7.15 turn off strict mode
On Aug 05, 2015, I answered the post Set sql_mode “blank” after upgrading to MySQL 5.6

In my answer, I explained how Oracle created an additional my.cnf called /usr/my/cnf.

It has this (keep in mind this is MySQL 5.6 I am answering back then)

# For advice on how to change settings please see
# http://dev.mysql.com/doc/refman/5.6/en/server-configuration-defaults.html

[mysqld]

# Remove leading # and set to the amount of RAM for the most important data
# cache in MySQL. Start at 70% of total RAM for dedicated server, else 10%.
# innodb_buffer_pool_size = 128M

# Remove leading # to turn on a very important data integrity option: logging
# changes to the binary log between backups.
# log_bin

# These are commonly set, remove the # and set as required.
# basedir = .....
# datadir = .....
# port = .....
# server_id = .....
# socket = .....

# Remove leading # to set options mainly useful for reporting servers.
# The server defaults are faster for transactions and fast SELECTs.
# Adjust sizes as needed, experiment to find the optimal values.
# join_buffer_size = 128M
# sort_buffer_size = 2M
# read_rnd_buffer_size = 2M

sql_mode=NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES


I guess some developer just gave up putting it in the code and slapped up a configure file as some demented shortcut. 

If you have such a file, please comment out the last line and restart mysqld.

MySQL 5.7's default value for sql_mode is as mentioned

ONLY_FULL_GROUP_BY STRICT_TRANS_TABLES NO_ZERO_IN_DATE NO_ZERO_DATE ERROR_FOR_DIVISION_BY_ZERO NO_AUTO_CREATE_USER NO_ENGINE_SUBSTITUTION


UPDATE 2017-04-27 17:23 EDT

I just ran mysqld --help --verbose | head -13 and got this

$ mysqld --help --verbose | head -13
2017-04-27T21:17:13.237941Z 0 [Warning] Changed limits: max_open_files: 1024 (requested 5000)
2017-04-27T21:17:13.238060Z 0 [Warning] Changed limits: max_connections: 214 (requested 500)
2017-04-27T21:17:13.238065Z 0 [Warning] Changed limits: table_open_cache: 400 (requested 2048)
mysqld  Ver 5.7.17-log for Linux on x86_64 (MySQL Community Server (GPL))
Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Starts the MySQL database server.

Usage: mysqld [OPTIONS]

Default options are read from the following files in the given order:
/etc/my.cnf /etc/mysql/my.cnf /usr/etc/my.cnf ~/.my.cnf


Please run mysqld --help --verbose 2&gt;/dev/null | head -13 | tail -1

Check every file mentioned

UPDATE 2017-04-29 19:03 EDT

The /usr/my.cnf trick I mentioned in Set sql_mode “blank” after upgrading to MySQL 5.6 comes from a post written by Morgan Tocker under the heading "Changes in MySQL 5.6". This bait-and-switch method for sql_mode may not apply in this case since this is MySQL 5.7.

As I mentioned in my earlier comment, I have successfully sql_mode by adding this to /etc/my.cnf

[mysqld]
sql_mode=''


You should not have to restart mysqld. You login to mysql and run

SET GLOBAL sql_mode='';
SELECT @@GLOBAL.sql_mode;


This will set incoming connections to a blank sql_mode

This will not change the sql_mode of currently established connections.

Restarting mysqld will guarantee the all incoming connections will have it blank.

If you cannot restart mysqld, you must restart your app/web servers and make them disconnect. Then,m restart your app/web servers to establish connections.

Please try this and let us the results.