allow underscores inside of network names (#5652)
allow underscores inside of network names (#5652)

JIRA Issues: MARATHON-7848
gsantovena/marathon,janisz/marathon,guenter/marathon,meln1k/marathon,guenter/marathon,guenter/marathon,janisz/marathon,guenter/marathon,janisz/marathon,mesosphere/marathon,mesosphere/marathon,gsantovena/marathon,gsantovena/marathon,gsantovena/marathon,meln1k/marathon,meln1k/marathon,janisz/marathon,meln1k/marathon,mesosphere/marathon,mesosphere/marathon,meln1k/marathon,guenter/marathon,janisz/marathon,mesosphere/marathon,gsantovena/marathon