Added autosign and dns_alt_names parameters

Hi,

These changes would add 2 parameters:
- dns_alt_names: comma-separated list of alternative DNS names (doc: https://docs.puppetlabs.com/references/latest/configuration.html#dnsaltnames)
- autosign: a configuration file or a custom policy executable (doc: https://docs.puppetlabs.com/references/latest/configuration.html#autosign)

Note that you still need to add the custom policy executable via another method.
We embed a pre-shared key as a CSR attribute and a shell script on the master verifies if the CSR received from the agent contains this key.
The name/location of the script is passed as the autosign param.