cmdlib: Create wrapper for GenerateDiskTemplate
cmdlib: Create wrapper for GenerateDiskTemplate

Create a wrapper (_GenerateDiskTemplateWrapper) for the GenerateDiskTemplate
function. This wrapper includes all the necessary functions (in the context of
LUInstanceSetParams) that collect the arguments for GenerateDiskTemplate.

This wrapper is created as preparatory work for the patch regarding the
ownership of file-based instance disks.

Signed-off-by: Alex Pyrgiotis <6383a799dda2258eec8fa26a3ce2ec8a7d552341@grnet.gr>
Signed-off-by: Aaron Karper <99cc1031546340b2b8ced5fe338b5890677a56ca@google.com>
Reviewed-by: Aaron Karper <99cc1031546340b2b8ced5fe338b5890677a56ca@google.com>

yiannist/ganeti,mbakke/ganeti,andir/ganeti,ganeti/ganeti,leshchevds/ganeti,bitemyapp/ganeti,leshchevds/ganeti,grnet/snf-ganeti,yiannist/ganeti,andir/ganeti,bitemyapp/ganeti,dimara/ganeti,grnet/snf-ganeti,dimara/ganeti,onponomarev/ganeti,onponomarev/ganeti,mbakke/ganeti,andir/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti/ganeti,ganeti/ganeti,mbakke/ganeti