Rework environment configs and plugin activation

[rpeterman-gp]

Goals

• Make configs Pantheon agnostic by not depending on Pantheon environment variables to determine environment type.
  • Add logic to support host specific configs to anticipate hosting changes.
• Utilize mu-plugins plugin to configure active plugins for all environment types.
• Utilize wp-cfm for environment type agnostic settings (eg. settings that should apply to all env).
• Remove use of wp-cfm-path plugin.