Prevented webui from displaying unset drain config field.
Prevented webui from displaying unset drain config field.

This field might not be present and displayed as NaN in that case. With
this patch the field is unconditionally hidden.

Review: https://reviews.apache.org/r/71181/

dforsyth/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,reneploetz/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,dforsyth/mesos,kaysoky/mesos,reneploetz/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,Gilbert88/mesos,dforsyth/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos