Remove the usage of 'which' (following dib)

diskimage-builder deprecated the usage of which (dib-lint marks its
occurrences as error by default):
https://review.openstack.org/#/c/466063/

Replace all the occurences of 'which' with 'type -p'.
Also, remove the (now useless) installation of which from
the disable-firewalld element.

Change-Id: I00bda062767070e46133cc800e82c9470923f572

elements/disable-firewall/element-deps

@@ -1 +0,0 @@
-package-installs

elements/disable-firewall/package-installs.yaml

@@ -1,2 +0,0 @@
-which:
-  phase: post-install.d

elements/disable-firewall/post-install.d/30-disable-firewalld

@@ -8,7 +8,7 @@ set -o pipefail
 
 case "${DISTRO_NAME}" in
     fedora | centos | centos7 | rhel | rhel7 )
-        if which systemctl && [[ $(systemctl --no-pager list-unit-files firewalld) =~ 'enabled' ]]; then
+        if type -p systemctl && [[ $(systemctl --no-pager list-unit-files firewalld) =~ 'enabled' ]]; then
             systemctl disable firewalld
         fi
     ;;

elements/disable-firewall/post-install.d/31-disable-ip6tables

@@ -14,7 +14,7 @@ esac
 
 case "${DISTRO_NAME}" in
     fedora | centos | rhel | rhel7)
-        if which service; then
+        if type -p service; then
             service ip6tables save
             chkconfig ip6tables off
         fi

elements/disable-firewall/post-install.d/32-disable-iptables

@@ -14,7 +14,7 @@ esac
 
 case "${DISTRO_NAME}" in
     fedora | centos | rhel | rhel7)
-        if which service; then
+        if type -p service; then
             service iptables save
             chkconfig iptables off
         fi

elements/sahara-version/root.d/90-sahara-version

@@ -7,7 +7,7 @@ set -eu
 set -o pipefail
 
 function _get_version_package {
-    if which pbr > /dev/null; then
+    if type -p pbr > /dev/null; then
         local version=$(pbr info $1 | awk '{print $2}')
         echo "${version}"
     fi