841013 - Allow same name distributions in changeset

If you add two distributions with same name to a changeset it should be
allowed if they come from different products.
844389 - unsuccessful repo deletion rollbacking

If pulp was is unavailable during repository deletion a process failed
without rollbacking changes in candlepin. Now we delete content in
candlepin in such case so there is no inconsistency and you can try to
add repository again.

Also we display a better error notification to user instructing him to
try again. All backtrace is however stored in notification.

One minor fix regarding elastic search indexed models. Some strings can
be nil so we must check their presence, not their length.
851331 - Add organization label attribute

Provider JSON now includes organization_label attribute so it can be
used in other actions. No need to list all organizations to find out
label for given organization id anymore.
Merge pull request #1542 from ares/bug/841013-distro_unique_validation

841013 - Allow same name distributions in changeset
Merge pull request #1537 from ares/bug/844389-repository_deletion_and_creation

844389 - unsuccessful repo deletion rollbacking
Merge pull request #1544 from ares/bug/851331-provider_organization_id_format

851331 - Add organization label attribute
cli completion - fixed import path to 'fix_io_encoding'

The library structure has changed and the function was moved into another directory.
Merge pull request #1550 from tstrachota/cli_fix

cli completion - fixed import path to 'fix_io_encoding'