[disk] add a regex option to exclude disks based on mountpoint

[rhwlo]

summary

This changeset adds an exclude_mountpoint_re option the disk integration configuration and corrects a small typo in the disk integration configuration template.

motivation

Nagios’s check_disk plugin has a -i option to ignore paths or partitions based off of regular expressions, and this will add the same functionality to Datadog.

testing

I’ve added tests to mock/test_disk.py alongside the other disk tests to verify that the regexes exclude what we want and nothing else.

[olivielpeau]

Thanks @rhwlo for the PR, we'll be sure to review and test it soon. At first glance it looks great!

[remh]

That looks great, thanks!