gpg key error on CentOS 7 with default params

[wyardley]

Acceptance test has the error below.

It seems like it could be related to this (old) issue:

grafana/grafana#3126
https://community.grafana.com/t/repository-public-key-centos/687
etc.

I can't see, absent disabling the (default) repo source, how this could work. It's also confusing that 'testing' repo (which seems to have roughly the same version) installs the old version by default. I'll submit a separate PR for that.

Error: Could not update: Execution of '/bin/yum -d 0 -e 0 -y install grafana-2.5.0-1' returned 1: warning: /var/cache/yum/x86_64/7/grafana/packages/grafana-2.5.0-1.x86_64.rpm: Header V4 RSA/SHA1 Signature, key ID 3dd18bdd: NOKEY


The GPG keys listed for the "grafana repo" repository are already installed but they are not correct for this package.
Check that the correct key URLs are configured for this repository.

[wyardley]

Probably setting the default version to a more modern version would be a better fix.

[wyardley]

Closing via #61