MySQL cpu at 400% on c5.xlarge box
Remove these for a start, they are almost certainly counterproductive:

table_open_cache = 256
join_buffer_size = 1M
thread_concurrency = 16


There is also a conspicuous absence of innodb_buffer_pool_size being set.

Configuration isn't going to be the root cause of your problem, though - you need to identify what queries are eating all of your CPU and address those specifically, either by rewriting them or by making sure that appropriate indexes for them exist.