debian: use os-svc-enable to enable apache

apache service is called “httpd” on RHEL/Fedora and apache2 on
Debian/Ubuntu/Fedora.

As pointed by Ben Nemec, os-svc-enable is best way to deal with this
complexity and thus support all the operating system properly.

This commit requires change I125c46bebb0342ee3bdb957116e7c7e8e27a2ecc
on diskimage-builder to be able to enable apache with os-svc-enable.

Change-Id: I4720b61eb0fd15e539564edd25843f8332478abf
Merge "debian: use os-svc-enable to enable apache"