Abstract away virtualenv and CKAN config on the command-line

# What

Have a thin-wrapper around the paster CLI and pip commands that:
* runs them in the appropriate python virtual environment
* finds the CKAN config file (for paster CLI)
* find the plugin (for paster CLI)

# Examples

Existing:

$ . /usr/lib/ckan/default/bin/activate
(pyenv) $ pip install -r pip-requirements.txt

Replacement:

$ ./ckan_pip install -r pip-requirements.txt

Existing:

(pyenv) $ paster --plugin=ckanext-harvest harvester initdb --config=/etc/ckan/default/production.ini

Replacement:

$ ./ckan_manage harvester initdb

# Why

The python virtual environment causes a substantial proportion of CKAN support questions. Forget to activate when running any python command and you get a weird error. Worse, forget to activate it when running any 'pip install' command and you install stuff in the wrong place and it gets really confusing.

The CKAN config file is not confusing, but it is a faff to remember and verbose to document.

The plugin is annoying to have to remember and verbose to document.

The issue names the paster CLI, pip commands, the CKAN virtual environment, and the CKAN config and plugin arguments, but no implementation files or tests. Start by checking the current CKAN repository and its Discussions, since this ideas repository is deprecated, then determine where command wrappers belong. Done means both example workflows run without manual virtualenv, config-path, or plugin handling.

Written by the indexing model from the issue text.