Added $independent_key option.

Stores the keys with separated keys like debian, instead of store all the keys in a unique file.

Debian example:

# apt-key list
/etc/apt/trusted.gpg.d//debian-archive-squeeze-automatic.gpg
------------------------------------------------------------
pub   4096R/473041FA 2010-08-27 [caduca: 2018-03-05]
uid                  Debian Archive Automatic Signing Key (6.0/squeeze) <ftpmaster@debian.org>

/etc/apt/trusted.gpg.d//debian-archive-squeeze-stable.gpg
---------------------------------------------------------
pub   4096R/B98321F9 2010-08-07 [caduca: 2017-08-05]
uid                  Squeeze Stable Release Key <debian-release@lists.debian.org>

/etc/apt/trusted.gpg.d//debian-archive-wheezy-automatic.gpg
-----------------------------------------------------------
pub   4096R/46925553 2012-04-27 [caduca: 2020-04-25]
uid                  Debian Archive Automatic Signing Key (7.0/wheezy) <ftpmaster@debian.org>

/etc/apt/trusted.gpg.d//debian-archive-wheezy-stable.gpg
--------------------------------------------------------
pub   4096R/65FFB764 2012-05-08 [caduca: 2019-05-07]
uid                  Wheezy Stable Release Key <debian-release@lists.debian.org>

/etc/apt/trusted.gpg.d//puppetlabs-keyring.gpg
----------------------------------------------
pub   4096R/4BD6EC30 2010-07-10 [caduca: 2016-07-08]
uid                  Puppet Labs Release Key (Puppet Labs Release Key) <info@puppetlabs.com>

root@:~#