Update mysql.py

added the 'alter_time' column to the table 'salt_returns'
This column is convenient for us filtering SQL statements in a given time period and help us determine whether a statement execution success on some minions.
Update mysql.py

modify ''alter_time' TIMESTAMP DEFAULT CURRENT_TIMESTAMP' to ''alter_time' TIMESTAMP DEFAULT CURRENT_TIMESTAMP,'
Update mysql.py

change ' to `
Merge pull request #9006 from linuxme/master

 add the 'alter_time' column to the table 'salt_returns'