Fix "st2 action-alias execute" CLI command

[Kami]

I tried to use StackStorm after quite a long time and in the first 5 minutes I encountered two exceptions / bugs (like in the good old days - guess I'm always the "lucky" one :D).

There are two bugs in st2 action-alias execute command so it doesn't work.

1. If user query param is not specified, it will fail with the error below due to API returning an error due to type mismatch (out of the box behavior with st2-docker).

# -------- begin 140132975715104 request ----------
curl -X POST -H  'User-Agent: python-requests/2.23.0' -H  'Accept-Encoding: gzip, deflate' -H  'Accept: */*' -H  'Connection: keep-alive' -H  'X-Auth-Token: XXXX' -H  'content-type: application/json' -H  'Content-Length: 61' --data-binary '{"command": "foo=bar", "user": null, "source_channel": "cli"}' http://st2api:9101/aliasexecution/match_and_execute
# -------- begin 140132975715104 response ----------
{
    "faultstring": "None is not of type 'string'"
}

2. Result for match_and_execute API operation is not correctly de-serialized in the client so client returns empty object and the code throws an exception. API returns a container object with a list so we need to handle that correctly.

Traceback (most recent call last):
  File "/opt/stackstorm/st2/lib/python3.6/site-packages/st2client/shell.py", line 408, in run
    func(args)
  File "/opt/stackstorm/st2/lib/python3.6/site-packages/st2client/commands/action_alias.py", line 127, in run_and_print
    print("Matching Action-alias: '%s'" % execution.actionalias['ref'])
AttributeError: 'ActionAliasExecution' object has no attribute 'actionalias'

This PR fixes both issues.

TODO

• Add tests (no idea when I'll get a chance to do it, but for sure we need tests because I assume there are not otherwise they would have caught this bug)

[Kami]

Adding direct st2client tests will be a bit of a pain so to begin with, I added in-direct test for the API endpoint itself (5df4e8e - similar to the ones in #5141) which verify that API endpoint indeed returns a list with specific key present.

[Kami]

OK, I couldn't stand not having any tests for such a basic functionality so I also added direct tests in 19e2206 (still not ideal since response is mocked, but that's the pattern we use for CLI tests :/).

I also pushed a small related improvements which such make troubleshooting CLI tests easier - 75a114c.

[cognifloyd]

Wow. I didn't know about st2 action-alias execute. Awesome.
Fixes and tests lgtm

[Kami]

I'm also working on related functionality - execute-and-format which will run the execution which matches an alias and also format and print the result - this will make testing ChatOps easier and faster since you will be able to do it quickly on command line and no need to do it via chat.