support for virtual environments

Why you need it?

Currently there is only one global environment so you can be logged into one system at a time, and you have only one ~/.ops with its plugins

I want to add support to have multiple virtual environments

How it could be?

A virtual environent is (or should be) entirely defined by 2 variables

• OPS_HOME that sets the location of ~/.ops
• WSK_CONFIG_FILE that sets the location of the ~/.wskprops

It could be as simple as to check if in current directlry there is a .ops and in such a case to set OPS_HOME to the current directory and WSK_CONFIG_FILE to $OPS_HOME/.wskprops

Also it is useful to add a simple: ops -venv that will create a .ops and execute an update downloading in it task and prereqs.

Other related information

Check there are not somewhere implicit assumptions of ~/.ops in the task and the code that can break this new logic.
Update documentation.