Install Gradle quietly

When downloading Gradle to install inside the VMs used for testing, the
download progress logs do not play well with the Gradle progress logger
so we see garbage like:

==> centos-6: ==> Installing gradle
==> centos-6:
==> centos-6:
==> centos-6: %
==> centos-6:
==> centos-6: T
==> centos-6: o
==> centos-6: t
==> centos-6: a
==> centos-6: l
==> centos-6:
==> centos-6:
==> centos-6:
==> centos-6:
==> centos-6: %
==> centos-6:
==> centos-6: R
==> centos-6: e
==> centos-6: c
==> centos-6: e
==> centos-6: i
==> centos-6: v
==> centos-6: e
==> centos-6: d
==> centos-6:
==> centos-6: %
==> centos-6:
==> centos-6: X
==> centos-6: f
==> centos-6: e
==> centos-6: r
==> centos-6: d
==> centos-6:
==> centos-6:
==> centos-6: A
==> centos-6: v
==> centos-6: e
==> centos-6: r
==> centos-6: a
==> centos-6: g
==> centos-6: e
==> centos-6:
==> centos-6: S
==> centos-6: p
==> centos-6: e
==> centos-6: e
==> centos-6: d
==> centos-6:
==> centos-6:

This commit addresses this by setting curl to be silent and only show
errors. This instead gives:

==> centos-6: ==> Installing gradle
==> centos-6: Archive:  /tmp/gradle.zip

Relates #23460

Vagrantfile

@@ -266,8 +266,8 @@ def provision(config,
     }
 
     installed gradle || {
-      echo "==> Installing gradle"
-      curl -o /tmp/gradle.zip -L https://services.gradle.org/distributions/gradle-3.3-bin.zip
+      echo "==> Installing Gradle"
+      curl -sS -o /tmp/gradle.zip -L https://services.gradle.org/distributions/gradle-3.3-bin.zip
       unzip /tmp/gradle.zip -d /opt
       rm -rf /tmp/gradle.zip 
       ln -s /opt/gradle-3.3/bin/gradle /usr/bin/gradle