Add ALT_NEXUS_URL to create-netrc

This change is backwards compatible with existing system configs but
lets the user spec this env var which will take precedence.  This is
useful, for example, to point at nexus3.edgexfoundry.org if you have
NEXUS_URL=nexus.edgexfoundry.org.

Change-Id: Ibbba26670ab374fd98e1bf20f78ffff7f8b7c52a
Signed-off-by: Jeremy Phelps <jphelps@linuxfoundation.org>