Use apache2ctl after apache is installed.

Ensure that apache2ctl is only called if apache2 is being used when
setting up Keystone. This patch corrects a case where apache2ctl
is being called at the time lib/keystone is being sourced. This
error breaks attempts to deploy Keystone under apache unless
apache is already installed at the time stack.sh is run.

Change-Id: I02d426438fa1246effd68c6bb3c3358910ed4c05
Merge "Use apache2ctl after apache is installed."
Merge branch 'master' of https://github.com/openstack-dev/devstack