Fix CreateInstance new optional parameters
Fix CreateInstance new optional parameters

Some new paramenters of the CreateInstance opcode are optional (namely
kernel_path, initrd_path and hvm_boot_order) but their absence makes the code
crash. Fix this by initializing them to a default value if they're not present.

Reviewed-by: iustinp


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