Refactor Packages and AppCatalog pagination

Move function for getting list of Application packages to a separate
module - muranodashboard.api.packages, so both Packages and AppCatalog
views can reuse it. That `package_list()` function obtains packages
generator with a call to `muranoclient.packages.filter()` and paginate
the results, adding `has_more_data` flag (as it is done for Glance,
see http://tinyurl.com/lkbnsed).

Also move muranoclient-specific functions from
muranodashboard.environments.api to muranodashboard.api module.

Implements: blueprint app-catalog-pagination
Change-Id: Ic3a433c8467fda94ac7dd3800c29fc1c2b9f3464
Merge "Refactor Packages and AppCatalog pagination"