Set maximum concurrency by default

[mnapoli]

We could set a maximum concurrency by default at a lower setting than its default, which is 1000 parallel executions.

The goal for this would be to avoid scenarios where a lambda could be DOS by a malicious 3rd party and create a huge AWS bill in a short amount of time.

For example let's say the lambda executes in 100ms. Its execution could be trigger at maximum 10000 times per second (1000 lambdas in parallel). That means a bill of $1613.09 for a whole day of this (not counting API Gateway which is even more expensive!).

I guess we could set it explicitly to 20 for example (that would mean up to $30 per day). It could be by default in serverless.yml (https://github.com/mnapoli/bref/blob/master/template/serverless.yml). The feature seems to be implemented, see serverless/serverless#4555

Users would be of course free to change that value or remove the limit.

WDYT?