pylint: Add pylint into requirements.txt

pylint: Add pylint into requirements.txt

Pylint installation was part of vsperf installation scripts, which is
not following vsperf generic approach to add all required python
packages into requirements.txt. It also means, that the same
version of pylint is used for verification of all patches at given
server. This is causing issues in case, that different versions
of pylint are required to verify vsperf code, e.g. older version
might be required to verify patches of previous OPNFV release.

Pylint with exact version number was added into requirements.txt,
so correct version of pylint is used for verification of vspef code.

JIRA: VSPERF-563

Change-Id: I48c9cacd87f016e6047ed965728d3b8d63e6461f
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Richard Elias <richardx.elias@intel.com>