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