Validate account, url, key input

[tjfontaine]

With the advent of RBAC users are now accustomed to logging into the portal with Account/SubUser but some commands like listimages use SDC_ACCOUNT verbatim, and if the user inadvertently sets that to their portal credentials, the server is not able to respond with a message indicating a wrong user. Instead you get that the command isn't allowed.

Instead we should be validating the input for account, user, url, and key to make sure they contain appropriate information. In the case that inappropriate information is provided we should bail with guidance.