ci: lib: Configure SSH key for Ansible

In order for Jenkins to apply configuration policy on machines that are
not members of the *pyrocufflink.blue* domain, it needs to use an SSH
private key for authentication.

vars/applyConfigPolicy.groovy

@@ -61,6 +61,7 @@ def stageRemountRW(limit) {
                 playbook: 'remount.yml',
                 limit: limit,
                 become: true,
+                credentialsId: 'jenkins-cfgmgmt',
                 vaultCredentialsId: 'ansible-vault',
                 extraVars: [
                     remount_state: 'rw',
@@ -78,6 +79,7 @@ def generateStages(stages) {
                 ansiblePlaybook \
                     playbook: playbook,
                     become: true,
+                    credentialsId: 'jenkins-cfgmgmt',
                     vaultCredentialsId: 'ansible-vault',
                     extras: '--diff',
                     skippedTags: 'install'
@@ -94,6 +96,7 @@ def stageRemountRO(limit) {
                 playbook: 'remount.yml',
                 limit: limit + ':!rw-root',
                 become: true,
+                credentialsId: 'jenkins-cfgmgmt',
                 vaultCredentialsId: 'ansible-vault',
                 extras: '--diff'
         } else {