Enable "stress" tests to be run on demand

Enable "stress" tests to be run on demand

The "stress" test can be run in different ways, depending on:

  - the tested scenario (authoritative, recursive),
  - the operating system used (Linux, FreeBSD),
  - the architecture used (amd64, arm64).

Currently, all supported "stress" test variants are automatically
launched for all scheduled pipelines and for pipelines started for tags;
there is no possibility of running these tests on demand, which could be
useful in certain circumstances.

Employ the "only:variables" key to enable fine-grained control over the
list of "stress" test jobs to be run for a given pipeline.  Three CI
variables are used to specify the list of "stress" test jobs to create:

  - BIND_STRESS_TEST_MODE: specifies the test mode to use; must be
    explicitly set in order for any "stress" test job to be created;
    allowed values are: "authoritative", "recursive",

  - BIND_STRESS_TEST_OS: specifies the operating system to run the test
    on; allowed values are: "linux", "freebsd"; defaults to "linux", may
    be overridden at pipeline creation time,

  - BIND_STRESS_TEST_ARCH: specifies the architecture to run the test
    on; allowed values are: "amd64", "arm64"; defaults to "amd64", may
    be overridden at pipeline creation time.

Since case-insensitive regular expressions are used for determining
which jobs to run, every variable described above may contain multiple
values.  For example, setting the BIND_STRESS_TEST_MODE variable to
"authoritative,recursive" will cause the "stress" test to be run in both
supported scenarios (either on the default OS/architecture combination,
i.e. Linux/amd64, or, if the relevant variables are explicitly
specified, the requested OS/architecture combinations).