* Make dev project configurable
We should not hardcode our dev project in the public config file.
* Remove the use of .ext when using external properties
They are only needed when defining properties.
* Improve error handling for environment property
Improve the error messages that we get for a bad or missing environment
property. Move the property processing into the main build so that we do it
only once and configure all of the appengine deployment tasks to check that a
project has been defined and print a friendly error if it hasn't.
Note that even if the check isn't configured, this change prevents deployment
because the gcpProject will be set to null. It just won't print as useful an
error message.
Tested: ran appengineDeployAll with and without the environment property, ran
"build" to verify that we don't get any complaints for non-deployment targets.
* Changes for review.
* Changes for review
* Changes for review.
* Changes for review.
