Feature Request: Implement requirements check

[galenemery]

Building upon this PR: chef-boneyard/opscode-omnibus#104

There are several things that need to be correct/implemented on a server before we can install Chef Server. Currently there is no pre-flight or requirements check. This often results in unclear error messages and undelightful install process. Particularly in secure environments.

Please implement checks for the items in the requirement doc: https://docs.getchef.com/server/install_server_pre.html

Additionally, we should test if the Chef Server is in an offline or proxy mode, and needs additional config to reach the internet.

[marcparadise]

Note that in addition to requirements checks in preflight as specified in the link above, we should also look into package script checks that can perform very minimal sanity checks (free disk space > 2GB at /opt, > 2 GB at /var/opt and /var/log, free memory > 1G) prior to allowing the actual package installation to proceed.

Even though this is less than the actual requirements, we know it's enough to allow the package to get installed and any comprehensive preflight checks to be run - which includes varying requirements based on installation type.

[sdelano]

This is something that we'd like to implement, and the first version of which will be just creating a framework for running these tests via RSpec with ServerSpec, or the Audit Mode features in Chef 12. @stevendanna can you link to the branch that you have that takes a stab at this?

[tas50]

PG needs for preflight: #275

[tas50]

HOME env var: #116

[tas50]

More PG: #3040

[tas50]

IP route: #2777