Add GitHub workflows for both specification and acceptance tests

[jmciver]

The following commit closely mirrors CI execution principles used in the project's Travis-CI environment. Puppet 7 specification support and acceptance testing has been added. The following acceptance tests pairs have been configured:

centos-7 puppet5
centos-7 puppet6
centos-7 puppet7
centos-8 puppet5
centos-8 puppet6
centos-8 puppet7
debian-8 puppet5
debian-9 puppet6
fedora-30 puppet6
fedora-30 puppet7
fedora-31 puppet6
fedora-31 puppet7
fedora-32 puppet6
fedora-32 puppet7
fedora-33 puppet6
fedora-33 puppet7
ubuntu-1604 puppet5
ubuntu-1604 puppet6
ubuntu-1804 puppet5
ubuntu-1804 puppet6

Three functionality omissions exist:

• The official CentOS 6 Docker Hub container has a YUM configuration error and thus has been omitted from this commit (todo).
• RHEL8 does not have an official Docker Hub container and will require further research.
• Automatic releasing of this module to Puppet Forge has not been implemented in the GitHub actions workflow.

A minor fix has been applied to the fedora-30 node-set specification, which should have been added in commit ID d418180.

[jmciver]

The following URL provides an example of the output: https://github.com/jmciver/sgnl05-sssd/actions/runs/699926845