Get jenkins config only with the key exists

Get jenkins config only with the key exists

Handle config parser correctly which defaults to "[jenkins]" section
when no server is pased. This fixes the issue with checking if the key
exists in the configuration read before reading the key-value.

The issue is reproduceable by running `lftools jenkins plugins --help`
or `tox -e docs`, with the ``jenkins.ini`` file having no "[jenkins]"
section.

Use SafeConfigParser which is a saner version for parsing config files.

Issue: RELENG-2120
Change-Id: I274ac46cf7a1d03cf7c3c6fdaec6094b5e02aa42
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>