Plugin installation blindly upgrading deps

[Christopher Powell]

When installing opencafe plugins, the EnginePluginManager executes a 'pip install --upgrade'. This has the unfortunate side effect of recursively and blindly upgrading all python packages, even if the dependency requirements have already been met. It should execute an 'only-if-needed' upgrade.

https://github.com/openstack/opencafe/blob/master/cafe/configurator/managers.py#L655
https://pip.pypa.io/en/stable/user_guide/#only-if-needed-recursive-upgrade

How to reproduce, installing opencafe with the http plugin will break docker-compose (requests<2.8,>=2.6.1).

(venv)root@server:/root# pip install docker-compose

(venv)root@server:/root# pip freeze
backports.ssl-match-hostname==3.4.0.2
docker-compose==1.5.2
docker-py==1.6.0
dockerpty==0.3.4
docopt==0.6.2
enum34==1.1.1
functools32==3.2.3.post2
jsonschema==2.5.1
PyYAML==3.11
requests==2.7.0
six==1.10.0
texttable==0.8.4
websocket-client==0.34.0

(venv)root@server:/root# pip install opencafe

(venv)root@server:/root# cafe-config plugins install http

(venv)root@server:/root# pip freeze
backports.ssl-match-hostname==3.4.0.2
cafe-http-plugin==0.0.1
docker-compose==1.5.2
docker-py==1.6.0
dockerpty==0.3.4
docopt==0.6.2
enum34==1.1.1
functools32==3.2.3.post2
jsonschema==2.5.1
opencafe==0.2.4
PyYAML==3.11
requests==2.9.0
six==1.10.0
texttable==0.8.4
websocket-client==0.34.0