Fix release-job.sh Nexus proxy var

Fix release-job.sh Nexus proxy var

Avoid the usage of project specific variables.
Do not use ODLNEXUSPROXY var, but instead use
a generalized variable.

Projects using lf-release-jobs need to make
sure they have the global variable NEXUSPROXY
added in Jenkins production and Jenkins sandbox
servers.
The value of this variable should be the URL to
the project's Nexus server.

Issue: RELENG-2131
Change-Id: Ia33c106c3c0ff86e57064deb74fb3863fcd90bdc
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>