Make Linux config files permissions less open

[loliee]

Prevent main & check configuration files to be read by everybody.
The reason is they can contains some secrets.

Before this modification we had something like that:

$ ls -lah /etc/datadog-agent/conf.d/mysql.d/conf.yaml
-rw-r--r-- 1 dd-agent dd-agent 657 Nov 30 15:15 /etc/datadog-agent/conf.d/mysql.d/conf.yaml
$ ls -lah /etc/datadog-agent/datadog.yaml
-rw-r--r-- 1 dd-agent dd-agent 215 Nov 30 15:09 /etc/datadog-agent/datadog.yaml

After it become:

$ ls -lah /etc/datadog-agent/datadog.yaml
-rw-r----- 1 dd-agent dd-agent 215 Dec  1 12:26 /etc/datadog-agent/datadog.yaml
$ ls -lah /etc/datadog-agent/conf.d/mysql.d/conf.yaml
-rw-r----- 1 dd-agent dd-agent 657 Dec  1 12:26 /etc/datadog-agent/conf.d/mysql.d/conf.yaml