Support multiple nexus sections in lftools.ini

Support multiple nexus sections in lftools.ini

[nexus] section is taken from -s "server" passed to release job.
The "https://" part of passed url is stripped before match.

example:
lftools nexus release -s https://nexus.example.com

Would require auth section in lftools.ini of
[nexus.example.org]
username=
password=

Issue: RELENG-2129
Issue: RELENG-1666
Change-Id: I64ab0a18a4d0e9060f1273f5acc1971fcffe497c
Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>