htools: split tryMGAlloc in two
htools: split tryMGAlloc in two

Currently, this function does both identify the best group for an
instance, and format the results (partially). The former functionality
is useful in other contexts too, so let's split the function in two.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

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