Add an option --force-failover
Add an option --force-failover

...to be added to gnt-group evacuate forcing evacuation by
means for failovers instead of migrations.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Thomas Thrainer <98bdeac947eb8ca18146e9ce7f092788ec242cd1@google.com>

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