packaging: use APP_NAME in /etc/exports comments

when configuring /etc/exports use APP_NAME instead of the
hardcoded value "rhev installer".

Change-Id: Ie7c343408d586f756251247da51a3412a5a9afdd
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=904149#c3
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>