In order to access property set during a flow you need to use the Mule Expression language.
Try using the following to access your property.
#[message.outboundProperties['certPath']]
Note: This works only when the property is accessed in the same flow.
The ant style access ${session.certPath}
is for loading properties from properties file.
Refer the following link for more details on MEL.
http://www.mulesoft.org/documentation/display/current/Property+Transformer+Reference
Hope this helps.