Integrate checking of md5 sum for public download links

[metmajer]

Add option to download an md5 file and, if one is defined, check the checksum via the get_url module's checksum parameter in tasks/install-dynatrace-wsagent-linux.yml (since Ansible 2.0).

Our public downloads on http://downloads.dynatracesaas.com offer a file $installer.md5 for each $installer. This should be reflected by default in variables dynatrace_wsagent_linux_installer_md5_file_name and dynatrace_wsagent_linux_installer_md5_file_url in defaults/main.yml.