Pulp

Validate absolute pathnames in remotes' URLs

Before this change, it was not possible to determine why did the synchronization fail when a user provided a seemingly valid URL. This commit also adds more relevant information to the error message.

Having set ALLOWED_EXPORT_PATHS to ["/tmp", "/home/vagrant/test"], the following error messages are shown:

$ pulp file remote create --name test --url file://error/vagrant/test/centos-7/PULP_MANIFEST
Error: {"url":["The path 'error/vagrant/test/centos-7/PULP_MANIFEST' needs to be an absolute pathname."]}

$ pulp file remote create --name test --url file:///error/vagrant/test/centos-7/PULP_MANIFEST
Error: {"url":["The path '/error/vagrant/test/centos-7/PULP_MANIFEST' does not start with any of the allowed import paths"]}

closes #9080