[JUJU-858] Add quality of life feature ensure application removal at return

[cderici]

Description

This is to make life easier by allowing the following pattern:

    await ops_test.model.applications[related_app.name].remove()
    # Block until it is really gone. Added after an itest failed when tried to redeploy:
    # juju.errors.JujuError: ['cannot add application "related-app": application already exists']
    await ops_test.model.block_until(lambda: related_app.name not in ops_test.model.applications)

to be written as:

    await ops_test.model.remove_application(related_app.name, block_until_done=True)

Fixes #656

QA Steps

tox -e integration -- tests/integration/test_application.py::test_app_remove_wait_flag

Notes & Discussion

Another option would be to do await ops_test.model.applications[related_app.name].remove(block_until_done=True), however, then it gets weird to ask the app.model to block_until the app.name is no longer in model.applications while the app object is being torn down.

[juanmanuel-tirado]

LG2M

[cderici]

$$merge$$