Implement a skeleton of 'cf assign-security-group"

[#74078880]
Rename some variables on Command interface

Single letter variable names! Not on my watch!
'cf assign-security-group' fails without a name

[#74078880]
'cf assign-security-group' expects an org and space

[#74078880]
Add simple API object to bind spaces to sec. groups

[#74078880]