letsencrypt: Obtain certificates automatically

Users should not have to obtain certificates manually after adding a domain the configuration. Let's Encrypt should try to automatically obtain certificates and notify the user of success of failure. This means that when domain configuration is changed in 'Config', 'Pagekite', 'Dynamic DNS' certificate should be obtained for newly added domain. When domain name is changed, the certificate should be obtained for changed domain name. This should also work when doing this configuration in first boot wizards.