Fixed grain matching on Amazon Linux

The osmajorrelease grain on Amazon Linux has only one element, and doesn't currently get matched correctly, resulting in the following error. This patch fixes that:

    - Rendering SLS "base:epel" failed: Jinja variable 'pkg' is undefined; line 36
    - 
    - ---
    - [...]
    - 
    - install_pubkey:
    -   file:
    -     - managed
    -     - name: /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
    -     - source: {{ salt['pillar.get']('epel:pubkey', pkg.key) }}    <======================
    -     - source_hash:  {{ salt['pillar.get']('epel:pubkey_hash', pkg.key_hash) }}
    - 
    - epel_release:
    -   pkg:
    -     - installed
    - [...]
    - ---