CP-36097 REQ-403 write-pems API
CP-36097 REQ-403 write-pems API

Motivation for this API is the following upgrade case:

We have a stockholm cluster, which is upgraded to the latest version of
the toolstack. Before enabling TLS verification, the cluster does not
have a pem file to use, so it will use xapi's.

However in order to enable TLS verification, it will need its own pem.
'write-pems' will introduce a pem to such a cluster.

Signed-off-by: Ben Anson <61f89ad796c1abd95282a5d95d051a3ac41f1c68@citrix.com>

xapi-project/message-switch,xapi-project/message-switch,xapi-project/message-switch