Document the --drbd-usermode-helper in man gnt-cluster
Document the --drbd-usermode-helper in man gnt-cluster

Signed-off-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

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