Fix Murano gate job

There were two patches which broke our gate:
1. https://review.openstack.org/#/c/80690/
   This one forces Devstack installation to fail if a project has dependencies
   not listed in the global-requirements file. Fix description:
   Devstack has two functions to install a project:
    * setup_develop
    * setup_package

   The first one does the requirements check and fails on Murano installation.
   The latter one doesn't perform any checks (it is called internally from the
   first function right after the requirements check was performed). We will
   use the latter one until all Murano requirements match global-requirements.

2. https://review.openstack.org/#/c/85032/
   This one introduced a nosetests plugin for MuranoPL. Somehow (I have no
   idea how) it breaks Tempest run in the gate. It causes our functional tests
   to behave wierdly, not recognise tempest config file and throw exception:
   "DuplicateOptError: duplicate option: use-syslog", see complete log at
   http://logs.openstack.org/13/85113/7/check/gate-murano-devstack-dsvm/4939248/console.html

   Fix description:
   Temporarily disable nosetests plugin until we figure out how to enable
   it and avoid Tempest problems in the gate.

Change-Id: I369f91fdabdc887d5ff3fb6e16d85a4d12de9f03