Increase exec timeout

The default value of the timeout parameter used in the exec type in Puppet is
300secs. freshclam takes longer than 300secs to complete at times. If this
happens when a box is brought up, the provisioning may fail. Even though
freshclam will re-run after a while once the box has booted, it is far better to
ensure that we get a successful output from freshclam during provision, rather
than an error relating to it timing out.

This pull request increases the timeout value from 300s to 600s.