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.