Add support for dnf as package manager

try to use it first and fall back to yum when not present.

Change-Id: I0001eb4b5ac2ea6deedef9548b2f51cd1adc38f3
Closes-Bug: #1585758

releasenotes/notes/dnf-support-413762b0af2887bc.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - bifrost now supports ``dnf`` as package manager on RedHat-based distros.
+    It tries to use it by default and falls back to ``yum`` when ``dnf`` is
+    not available.

scripts/install-deps.sh

@@ -48,9 +48,10 @@ elif [ -x '/usr/bin/apt-get' ]; then
               [wget]=wget
             )
     EXTRA_PKG_DEPS=()
-elif [ -x '/usr/bin/yum' ]; then
+elif [ -x '/usr/bin/dnf' ] || [ -x '/usr/bin/yum' ]; then
     OS_FAMILY="RedHat"
-    INSTALLER_CMD="sudo -H yum -y install"
+    PKG_MANAGER=$(which dnf || which yum)
+    INSTALLER_CMD="sudo -H ${PKG_MANAGER} -y install"
     CHECK_CMD="rpm -q"
     PKG_MAP=(
         [gcc]=gcc