SSC: Add support for creating appversion from variable

[rsenden]

Although probably low priority for now, allowing an appversion (and maybe also other entities) to be created from a variable could be a very powerful feature to copy application versions from one SSC instance to another:

for id in $(fcli ssc appversion list -o 'expr={id}\n'); do 
  fcli ssc appversion get ${id} --include-attrs --include-users ... --session source-ssc --store currentAppVersion 
  fcli ssc appversion create --from-var currentAppVersion --session target-ssc
done

This will require some additional though and work though, for example:

• Have options on the get command to include additional entities like attributes, users, bug tracker settings, ...
• On the create command, make <app>:<name> positional parameter optional if --from-var is specified, but this may confuse users as to when this positional parameter is required
• Any positional parameters/options on the create command should override the values available in the variable