Specify key algorithm in nsupdate plug-in's `del_cmd` and in `oo-diagnostics`

`oo-diagnostics`: Read DNS key algorithm from the broker's configuration and use it in `dns_bind_update_record`.

`plugins/dns/nsupdate/lib/openshift/nsupdate_plugin.rb`: Specify the DNS key algorithm in the `nsupdate` command constructed in the `del_cmd` method.

This PR forward-ports two commits from enterprise-server:

https://github.com/openshift/enterprise-server/pull/222
https://github.com/openshift/enterprise-server/pull/223