This patch replaces some hardcoded strings with their corresponding constant in `_GenerateDiskTemplate()`.
This patch replaces some hardcoded strings with their corresponding constant in `_GenerateDiskTemplate()`.

Reviewed-by: iustinp


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