Enable notifications for new dynamic uber_build workflow

This commit adds another build parameter, notify, which is populated via yaml
or via the NOTIFY= environment variable. This be used to set up the dynamic
jenkins task with a notification event for the status of the job. Usage:

rake pl:jenkins:uber_build NOTIFY=moses@puppetlabs.com

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Don't delete workspace if we don't have to