feat(examples): make mysql server name a required param

[vdice]

Previously, installing this bundle would most definitely result in an error, as the server name was hard-coded. The ARM deployment error would look something like:

Error: error deploying "mysql-azure-porter-demo-wordpress" in resource group "porter-test": error while waiting for deployment to complete: Code="DeploymentFailed" Message="At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/arm-debug for usage details." Details=[{"code":"BadRequest","message":"{\r\n  \"error\": {\r\n    \"code\": \"ServerNameAlreadyExists\",\r\n    \"message\": \"Specified server name is already used.\"\r\n  }\r\n}"}]

(tl;dr: \"message\": \"Specified server name is already used.\")

Therefore, in this PR, I propose to not only parameterize this value, but also not set a default and thus make it a required param when installing, say, via:

porter install --insecure -c amw --param server_name=porter-amw-vdice