Add task to check csf configuration

Previously, invalid configuration values were causing csf to
fail silently. This added task will make sure that ansible
fails early when csf configuration is invalid
Configuration check must occur after restarting csf
Use "command" module instead of shell

So that error codes can be inspected, e.g.
Fix all invalid configuration values
Add csf configuration check to security_checks role
Merge pull request #10 from CenterForOpenScience/fix-csf

Fix csf