Improve out-of-minors handling
Improve out-of-minors handling

Currently, the out-of-minors handling is not very good: though both MD
and DRBD functions for finding an unused minor can return None, only the
DRBD code checks for this case.

This patch improves this by making the functions _FindUnusedMinor raise
an error instead of None, and gets rid of the one manual check for the
return value.

Reviewed-by: imsnah


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