Replace bento/centos-7.3 with bento/centos-7

[emisca]

Centos 7.3 is failing to be provisioned as ansible is running a yum update and there are conflicts then.

I fixed it by switching to bento/centos-7 which is the latest minor release (currently 7.8):

diff --git a/Vagrantfile b/Vagrantfile
index 3938ead..5923d26 100644
--- a/Vagrantfile
+++ b/Vagrantfile
@@ -4,7 +4,7 @@
 # Main Vagrant Configuration for MongoSecurityPlaypen
 Vagrant.configure(2) do |config|
     # Install Centos 7.3
-    config.vm.box = "bento/centos-7.3"
+    config.vm.box = "bento/centos-7"

     # Set higher timeout for waiting for each VM to come up
     config.vm.boot_timeout = 600