You can do this, make your playbook part of a "Vault" and add a variable that contains your certificate content. Something like this:
---
- name: My playbook
  hosts: all
  vars:
    mycert: |
       blah
       blah
    - copy:
      content: "{{ mycert }}"
      dest: /mydecrypted.pem
    - name: Upload Cert
      copy: src=/home/ubuntu/mydecrypted.pem dest=/home/ubuntu/mydecrypteddest.pem
    - name: Delete decrypted cert
      file: path=/home/ubuntu/mydecrypted.pem state=absent