1119688: Improved error code usage in subman.

- Standardized most print & sys.exit calls with the system_exit
  abstraction.

- Standardized error code usage, using os.EX_* constants where appropriate:
  - EX_USAGE for improper usage of command options
  - EX_DATAERR for proper command usage with invalid provided data
  - EX_UNAVAILABLE for command options which are unsupported by the
    current server
  - EX_SOFTWARE for internal fatal errors
  - Others used somewhat liberally where seemingly appropriate

- Added error output to the status command:
  - 0 when the system is registered and no errors occur during the check
  - 1 when the system is not registered

- Updated the tests as necessary
Anchor the CDN regex in container plugin.
Merge pull request #1057 from candlepin/dgoodwin/anchor-regex

Anchor the CDN regex in container plugin.
Automatic commit of package [subscription-manager] release [1.13.7-1].
Merge pull request #1044 from candlepin/crog/1119688

1119688: Improved error code usage in subman.
Make repolib tag matching use model.find_content

repolib now uses model.find_content to match content
required tags and product provided tags. repolib and the
ostree and container plugins now use the same code for
this.

This also removes some of the verbose debug logging that
repolib previously had.