provider/azure: sanity check cert

- Don't read an unbounded amount of data
  from management-cert-path, because users
  are prone to specifying the wrong file.
- Verify the contents are PEM.

Fixes https://bugs.launchpad.net/ubuntu/+source/juju-core/+bug/1250007
Merge pull request #143 from axw/lp1250007-azure-sanity-check-cert

provider/azure: sanity check cert

- Don't read an unbounded amount of data
  from management-cert-path, because users
  are prone to specifying the wrong file.
- Verify the contents are PEM.

Fixes https://bugs.launchpad.net/ubuntu/+source/juju-core/+bug/1250007