Support command: "storage account connectionstring show [options] <name>"

Support command: "storage account connectionstring show [options] <name>" 
This PR is spilt from previous PR #1260.
The reason I need "connection-string" as a category instead of the command be just "azure storage account connection-string" is: 
I just want to follow the command naming rule like: azure storage + nouns + verb + parameters. So I add a verb as subcommand and use noun as category.