[Enhancement] Add support for multiple DNS names in SAN to dockertls.

[estenrye]

Use Case:
WHEN a common DNS name exists for all managers in a swarm (e.g. manager.nonprod.blah.io)
AND each node must support communication using the following DNS name examples
COMPUTERNAME
COMPUTERNAME.ACTIVE_DIRECTORY_DOMAIN
manager.nonprod.blah.io
AND each node in the swarm has DockerTLS enabled using the stefanscherer/dockertls-windows image
THEN dockerhost -H COMPUTERNAME ps should execute successfully
AND dockerhost -H COMPUTERNAME.ACTIVE_DIRECTORY_DOMAIN ps should execute successfully
AND dockerhost -H manager.nonprod.blah.io ps should execute successfully.

Suggested Implementation
Add a parameter to createCerts to supply an array of additional DNS names.
Modify the following line of createCerts to iterate over the array of additional DNS names and include them in the subjectAltName.
"subjectAltName = " + (($ipAddresses.Split(',') | ForEach-Object { "IP:$_" }) -join ',') + ",DNS.1:$serverName" | Out-File extfile.cnf -Encoding Ascii
Use an environment variable $env:ALTERNATIVE_NAMES to load the array of additional DNS names as a comma separated list.

[estenrye]

I intend to put up a pull request for this enhancement.

[StefanScherer]

@estenrye Thank you. Sounds reasonable. Seems like you also want to write a Cucumber integration test 👍 😃

[StefanScherer]

Closed with #183