bootstrap: Fix dnf usage on CentOS

CentOS 7.x (RHEL 7.x also) is based on Fedora 19 and it uses yum
as a default package manager.
14 jobs for bootstrap-centos-fix in 177 minutes and 48 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #396178
manifest

00:00:22

 
  Pre Build
passed #396180
cerbero deps android universal

01:11:41

passed #396179
cerbero deps fedora x86_64

00:50:28

passed #396184
gst-ios-12.2
cerbero deps ios universal

02:14:37

passed #396183
gst-macos-10.14
cerbero deps macos x86_64

00:56:23

passed #396181
cerbero deps windows x86

01:03:10

passed #396182
cerbero deps windows x86_64

00:43:25

 
  Build
passed #396186
cerbero android universal

00:09:21

passed #396187
cerbero cross win32

00:07:26

passed #396188
cerbero cross win64

00:05:07

passed #396185
cerbero fedora x86_64

00:05:15

passed #396190
gst-ios-12.2
cerbero ios universal

00:36:10

passed #396189
gst-macos-10.14
cerbero macos x86_64

00:10:26

 
  Integrate
passed #396191
cerbero android universal examples

00:06:34