Add libsasl2-dev package to jenkins slaves.

* modules/jenkins/manifests/slave.pp: The libsasl2-dev package seems to
already exist on older slave nodes, but those recently bootstrapped
are lacking it. Keystone's LDAP integration relies on this library.

Change-Id: Ib3df277ba0d90f5bd8e90a0276f148305d2841e7
Reviewed-on: https://review.openstack.org/19211
Reviewed-by: henry-nash <henryn@linux.vnet.ibm.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins