Add default value for copy_from timeout which is default_timeout in agent control body

AND
Correct default value of agent control "default_timeout"
Which is actually 30 seconds if not specified in agent control body.

https://github.com/cfengine/core/blob/3.6rc-build2/libpromises/cf3globals.c#L111

Redmine: https://dev.cfengine.com/issues/6104