Description of problem:

Pulp does not allow authentication with mongodb using username with an empty password if specified in server.conf and shows a traceback, though mongo allows access to pulp_database with an empty pass

Might be connected to BZ 1182335


Version-Release number of selected component (if applicable):
>> rpm -qa pulp-server
pulp-server-2.6.0-0.7.beta.fc20.noarch


Steps to Reproduce:
>>  mongo 
MongoDB shell version: 2.4.6
connecting to: test
> use pulp_database
switched to db pulp_database
> db.changeUserPassword("gena", "")
> exit
bye

>>  mongo pulp_database -u gena
MongoDB shell version: 2.4.6
connecting to: pulp_database
> db.repos.find()
{ "_id" : ObjectId("54b7c4cb99cca86045dd3fcb"), "_ns" : "repos", "content_unit_counts" : { "erratum" : 4, "package_category" : 1, "package_group" : 2 }, "description" : null, "display_name" : "zoo_repo", "id" : "zoo_repo", "last_unit_added" : ISODate("2015-01-15T13:47:14.329Z"), "last_unit_removed" : null, "notes" : { "_repo-type" : "rpm-repo" }, "scratchpad" : { "checksum_type" : "sha256" } }
> exit
bye

>> vi /etc/pulp/server.conf

username: gena
password:

>> for s in {qpidd,pulp_celerybeat,pulp_resource_manager,pulp_workers,httpd}; do sudo systemctl restart $s; done;

>> pulp-admin login -u admin -p admin
There was an internal server error while trying to access the Pulp application.
One possible cause is that the database needs to be migrated to the latest
version. If this is the case, run pulp-manage-db and restart the services. More
information may be found in Apache's log.

>> sudo -u apache pulp-manage-db
Database initialization failed: The server config specified username/password authentication but is missing either the username or the password
The server config specified username/password authentication but is missing either the username or the password
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/pulp/server/db/manage.py", line 124, in main
    connection.initialize(max_timeout=1)
  File "/usr/lib/python2.7/site-packages/pulp/server/db/connection.py", line 92, in initialize
    raise Exception(_("The server config specified username/password authentication but "
Exception: The server config specified username/password authentication but is missing either the username or the password



>> mongo 
MongoDB shell version: 2.4.6
connecting to: test
> use pulp_database
switched to db pulp_database
> db.changeUserPassword("gena", "genka")
> exit
bye
>> vi /etc/pulp/server.conf
username: gena
password: genka

>> for s in {qpidd,pulp_celerybeat,pulp_resource_manager,pulp_workers,httpd}; do sudo systemctl restart $s; done;

>> pulp-admin rpm repo list
+----------------------------------------------------------------------+
                            RPM Repositories
+----------------------------------------------------------------------+

Id:                  zoo_repo
Display Name:        zoo_repo
Description:         None
Content Unit Counts: 
  Erratum:          4
  Package Category: 1
  Package Group:    2