CLI: don't assume default channel

[MartinBasti]

Default channel is server configuration, CLI tool shouldn't change it

Signed-off-by: Martin Basti mbasti@redhat.com

Maintainers will complete the following section

• [x] Commit messages are descriptive enough
• [x] Code coverage from testing does not decrease and new code is covered
• n/a JSON/YAML configuration changes are updated in the relevant schema
• n/a Pull request has a link to an osbs-docs PR for user documentation updates

[MartinBasti]

RN

• CLI uses default channel defined by koji server ('container' channel was hard-coded before)