postmaster_address

[rblm]

Initial setup on my CentOS 7 guest fails without a postmaster_address definition in file 15-lda.conf. Suggest creating a variable to allow configuration.

[mprahl]

@rblm this role currently only supports Ubuntu. I'd love to have CentOS/RHEL support but I don't have the time to do this. Feel free to submit a PR that adds support.