Pre-install apache2

Apache2 is a dependency of openstack-dashboard which gets installed in
the install() function. However, prior to that we register configs
which does a check of the apache2 package version leading to a chicken
and egg race.

Pre-install apache2 in the bash install script to avoid this race.

Closes-Bug: #1799500

Change-Id: I1f73a6b0284ca4e0a4018bff73fd4f8a778c4db5