Allow alternate qbec app file name, instead of just qbec.yaml

[gotwarlost]

Because we have a restriction of the file always having to be named qbec.yaml people do weird things with component includes and excludes.

Say, they want to write functional tests deployed separately from service components, they create a fake environment called functional-test, and set up the functional test components as included here and all other components as excluded.

They rightly don't want to create separate deploy trees for main components and functional tests.

We could help them to do the right thing by making the qbec.yaml name configurable such that they write a qbec.yaml for main components and a qbec-functional.yaml for installing functional tests with a different component directory.

We could add a global param called --file with a short form of -f and an env var override of QBEC_FILE that are consulted for loading the correct app definition.

[gotwarlost]

The actual reason for supporting component includes and excludes is for experimentation with new components and selectively installing components. The above use case stretches this expectation by quite a bit.