Adopt a list of supported systems

[brianv0]

For guidance for developers and documentation, it'd be good if we could adopt a list of officially supported platforms and one blessed docker platform.

My Suggestions:

• Mac OSX - Most recent 2 versions, with the oldest release being deprecated at 6 months after current release. Users must have homebrew installed.
• Ubuntu LTS: Support current LTS release. (e.g. 16.04.1+, eventually 18.04.1+)
• CentOS: Support most recent DM combination (e.g. CentOS 7+SCL/devtoolset)
• NERSC Support for Cori

For Containers:

• Standardize on the supported Ubuntu LTS release

What this would mean today:

• Mac OS X 10.11, 10.12
• Ubuntu 16.04 LTS
• Centos 7 + devtoolset-6

[heather999]

Closing but we should revisit this