Add option to specify app-tag template

[harsimranmaan]

The app-tag works fine for ci/cd use cases but sometimes team have specific naming needs. Eg: On my team, we use user--resourceName tagging convention. While app-tag is used to provide similar behaviour by adding the the app-tag suffix, since it only supports suffix, it cannot be used for our use case. It would be nice to have a template option in qbec. Something like app-tag-template: user-{app-tag}-{name} would help address that dynamic naming conventions

[harsimranmaan]

Since the qbec.yaml param is named namespaceTagSuffix, this change would be backwards incompatible. Backwards Compatibility can be provided for a few releases to allow for migration to something like namespaceNameTemplate. This would simplify "## Usage pattern 2: mangle every object's name" at https://qbec.io/userguide/usage/branches-and-ci/#usage-pattern-2-mangle-every-object-s-name