ganeti.rpc: Read SSL certificate and key only once per request
ganeti.rpc: Read SSL certificate and key only once per request

There's no need to read the SSL certificate and key for every node
in a request. Also add a TODO for better error reporting.

Reviewed-by: amishchenko


dimara/ganeti,grnet/snf-ganeti,onponomarev/ganeti,dimara/ganeti,leshchevds/ganeti,onponomarev/ganeti,bitemyapp/ganeti,bitemyapp/ganeti,apyrgio/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,apyrgio/snf-ganeti,mbakke/ganeti,mbakke/ganeti,andir/ganeti,mbakke/ganeti,grnet/snf-ganeti,andir/ganeti,andir/ganeti,ganeti/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,leshchevds/ganeti,ganeti/ganeti,yiannist/ganeti,apyrgio/ganeti