Fix warning about invalid format in pip list

[nmaludy]

In recent versions of pip, the legacy format has been removed causing the following warning when running st2-run-pack-tests:

Installing dependencies from st2 repository...

Usage:   
  pip list [options]

option --format: invalid choice: 'legacy' (choose from 'columns', 'freeze', 'json')
Installing global pack test dependencies...
Installing pack-specific dependencies...
Installing pack-specific test dependencies..

This PR fixes the issue by using the freeze format instead of legacy.

[Kami]

Is freeze also supported in older versions of pip where we used legacy?

[laixintao]

looks good

[Kami]

Confirmed freeze format is also available in older versions of pip (we pin to 9.x in various places).

[laixintao]

I tested in 10.x and 18.x

[Kami]

@laixintao Thanks for confirming.

I need to wait with merging this PR (and other PRs) a bit until we sort out our CI issue - we spun up a new instance, but webhooks were not set up correctly so end to end tests are not running on Github commit / push.