Make a2dissite fail softly if the site is not enabled

a2dissite will return a non-zero error code if the site that is being
disabled is not currently enabled (that is, if the conf file for it does
not exist). This can happen during development if you've been messing
with files by hand. Rather than exploding out of a ./stack.sh, accept
the missing file as meaning "it's disabled" and carry one. The rpm
version of disable, which does not use a2dissite, does this already.

Change-Id: Ie5dfd42efdff4bdba5ffaa765af000dd8e1d596e
diff --git a/lib/apache b/lib/apache
index 20700d8..f88f1d3 100644
--- a/lib/apache
+++ b/lib/apache
@@ -186,7 +186,7 @@
 function disable_apache_site {
     local site=$@
     if is_ubuntu; then
-        sudo a2dissite ${site}
+        sudo a2dissite ${site} || true
     elif is_fedora || is_suse; then
         local enabled_site_file="$APACHE_CONF_DIR/${site}.conf"
         # Do nothing if no site config exists