Update default_settings to use environment variables.
Update default_settings to use environment variables.

Easier environment configuration when using docker to
run puppetboard.

grandich/puppetboard,grandich/puppetboard,johnzimm/puppetboard,mterzo/puppetboard,stoyansbg/puppetboard,mterzo/puppetboard,johnzimm/puppetboard,grandich/puppetboard,voxpupuli/puppetboard,stoyansbg/puppetboard,johnzimm/xx-puppetboard,grandich/puppetboard,voxpupuli/puppetboard,tparkercbn/puppetboard,tparkercbn/puppetboard,tparkercbn/puppetboard,tparkercbn/puppetboard,mterzo/puppetboard,johnzimm/puppetboard,johnzimm/xx-puppetboard,puppet-community/puppetboard,johnzimm/xx-puppetboard,johnzimm/xx-puppetboard,puppet-community/puppetboard,mterzo/puppetboard,voxpupuli/puppetboard,puppet-community/puppetboard,stoyansbg/puppetboard