Specify path for ceph::key Exec resources

The commands in the ceph::key Exec resources aren't fully qualified and causes
the following error when trying to use them:

Error: Failed to apply catalog: Validation of
Exec[/etc/ceph/ceph.test.keyring-touch] failed: 'touch
/etc/ceph/ceph.test.keyring' is not qualified and no path was specified. Please
qualify the command or specify a path. at
/etc/puppet/modules/ceph/manifests/key.pp:81 Wrapped exception: 'touch
/etc/ceph/ceph.test.keyring' is not qualified and no path was specified. Please
qualify the command or specify a path.

If a site wide default is set, then this won't be encountered, but that
shouldn't be required for this to work out of the box.

This commit specifies the path as "/usr/bin:/bin/:${path}".

Change-Id: I4aa422f7ddeb09568a597de6e5bbb2f0992560c7
Closes-Bug: 1314349