Change _GetWanted* to return names instead of objects
Change _GetWanted* to return names instead of objects

On closer look, all except one of the current users of _GetWantedNodes are
using only the name of the nodes and throw away the other attributes. It makes
sense to make this function return only the name list (as in the future this
might be faster than computing all attributes).

Reviewed-by: imsnah


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