Disable default olsrd key, forces creation of new one

This should satisfy https://github.com/opentechinstitute/commotion-feed/issues/16 and https://github.com/opentechinstitute/luci-commotion/issues/19, though we should still support a full range of serval keyring operations as in luci-commotion/issues/26.

To test on a node: `cd /etc/servald |serval id self` should give you something like `C6D124238A0F6E5BF6E674346DC798A2D840D78037AC79FF039C74E9446FD111`, which should be unique to each node.