If the user specifies long IDs, use them for all commands except for
deleting a key.  Need to use short IDs there because of an upstream
apt_key bug.  Fixed in apt_key 1.10 (fix is present in Ubuntu 16.04 but
not Ubuntu 14.0 or some Debians).

Fixes ansible#5237

Also:

* Some style cleanups
* Return a nice user error message if the key was not found on the
  keyserver
* Make file and keyring parameters type='path' so envars and tilde are
  expanded

When erasing a key, apt-key does not understand how to process subkeys.
This update explicitly checks that the key_id is no longer present and
throws an error if it is.  It also hints at subkeys being a possible
problem in the error message and the documentation.

Fixes ansible#5119

* More cleanups
* apt-key can be given a key id longer than 16 chars to more accurately
  define what key to download.  However, we can use a maximum of 16
  chars to verify whether a key is installed or not.  So we need to use
  different lengths for the id depending on what we're doing with it.

Fixes ansible#2622