Preserve --mobile-server setting across hot code pushes.

The server sends hot code push updates to mobile clients with ROOT_URL
and DDP_DEFAULT_CONNECTION_URL taken from the MOBILE_ROOT_URL and
MOBILE_DDP_URL environment variables. These are set by the main meteor
process when it starts the app runner.
Add test for --mobile-server propagating through hot code pushes
Merge branch 'mobile-settings-hcp' into devel