Fix registry auth by storing the string passed on the command line, and allowing for credential selection by normalizing on hostname. Also, remove remote ping calls from CmdPush and CmdPull.

Docker-DCO-1.1-Signed-off-by: Jake Moshenko <jake@devtable.com> (github: jakedt)
Merge pull request #4270 from DevTable/fixregistryauth

Fix registry auth and remove other hidden ping commands from client code