Use 'amp create' so that user can get prompts.

I adjusted 'amp create' so that it will run 'amp config' if amp isn't
configured, but in that case the user needs to respond to prompts on stdout,
so this makes the environment vars go to a file instead of stdout.
Make it so you don't need to specify db port.

When filling out the DSN for amp, I tend to leave the port out if I want to
use the default port. This fixes things so that it works.
A few more places that need support for MySQL port.

There were a few more places that weren't working if MySQL is not on the
default port. Note that this also needs a patch to bin/setup.sh in
civicrm-core.
drupal-demo/install.sh - Improve safety
Update DB environment variables:

 * Update the "build host-port" helper so that it doesn't create global var
 * Fix calls to mysqldump when omitting port (using "*_DB_ARGS" instead of manually building out list)