Commit for ticket #5755, issue https://github.com/ascom-au/ensurespeech-playbook/issues/10

vars/main.yml
* Removed the source download directories/packages and links, removed all references to build libraries, updated the dir_asterisk_config variable to point to /etc/asterisk

README.md
* Removed all references to asterisk/dahdi downloads, removed reference to global_config_dir variable

tasks/main.yml
* Removed installation of required libraries, removed creation of source directory, removed reference to libpri.yml file, removed build packages and source cleanup tasks

tasks/asterisk.yml
* Removed requires libraries, headers, source and make tasks, added asterisk/asterisk-core-sounds-en and asterisk-mp3 debian packages to installation

tasks/dahdi.yml
* Removed dahdi source, drivers, hardware and start tasks, added asterisk-dahdi debian package to installation

tasks/asterisk_configuration.yml
* Removed asterisk sounds relocation tasks, fixed reference to asterisk man page

tasks.dahdi_configuration.yml

* Added dahdi start task (relocated from task/dahdi.yml)

tasks/libpri.yml
* Removed redundant libpri.yml task

required-repos
* Added new file

required-packages
* Added new file containing asterisk and dahdi debian packages

required-keys
* Added new file