assert: add optional `msg` argument

Sample playbook when it may be useful:
```
- uri: return_content=yes url={{ api_base }}/action/{{ inventory_hostname }}
  register: output
- assert:
    msg: '{{ output.json | to_nice_json }}'
    that:
      - output.json.foo != "bar"
      - output.json.boo == 42
```
Adds an option to specify the glance endpoint type

Some environments that utilize an SSL terminator with a self-signed
certificate can use the publicURL without getting certificate
verify errors.  This allows using the internalURL with in my case
is HTTP and not HTTPS.

Closes issue: #8057
Merge branch 'assert-with-msg' of https://github.com/darkk/ansible into darkk-assert-with-msg
Tweak to only show msg on assert failure if set
Merge branch 'darkk-assert-with-msg' into devel
Merge branch 'bug/8057' of https://github.com/ericwb/ansible into ericwb-bug/8057
Merge branch 'ericwb-bug/8057' into devel
Merge remote-tracking branch 'upstream/devel' into devel