MB-10083 Add support for SSL_CERTS_REFRESH memcached command.

Change-Id: I3bbca5b85e7014caab124e9605ea2a49cd1eefde
Reviewed-on: http://review.couchbase.org/33545
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10083 Generate memcached certificate and private key files.

In when compat mode is 3.0, certificate file contains both node and
cluster certificates and private key file contains node's private
key. Otherwise certificate file contains cluster certificate and
private key file contains cluster private key.

Change-Id: I08be6e05a4da9c81ff09bd6f76e70ccaa0ddc4ce
Reviewed-on: http://review.couchbase.org/33512
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>