Add mercurial package to jenkins slaves.

* modules/jenkins/manifests/slave.pp: The mercurial package seems to
already exist on older slave nodes, but those recently bootstrapped
are lacking it. Some tests specify HG repositories in their
test-requires and possibly elsewhere, so this will make it available
to them.

Change-Id: Iafc012838745d4e740c360b0cfee8879fc6b5b3c
Reviewed-on: https://review.openstack.org/19034
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins