MySQL 5.7 doesn't start after installation on CentOS 6.7
I faced the same issue myself and I successfully resolved it.
What needs to be done is:
Open the my.conf file in vi editor and add a line under [mysqld]:

explicit_defaults_for_timestamp = 1


This will fix the first issue.
Now check the access rights of the location pointed by datadir field and make sure mysql can access it. 
Worst case rename the location it points to. eg:

datadir=/var/lib/mysql1