Description of problem:
2 scenarios of Permission denied when exporting repo:

1) exporting to a non-existing repo returns Permission denied error, but creates a specified repo and completes an export 
2) exporting to an existing repo returns Permission denied error, and doesn't export anything

Version-Release number of selected component (if applicable):
>> rpm -qa | grep pulp-server
pulp-server-2.5.0-0.18.rc.fc20.noarch

How reproducible:
always

Steps to Reproduce:

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

Id:                  gena
Display Name:        gena
Description:         None
Content Unit Counts: 
  Rpm: 2

>> ls -l | grep tmp
drwxrwxrwt.   8 apache apache  4096 Nov 13 13:27 tmp

>> ls -l /tmp
total 4
srwxrwxrwx. 1 mongodb mongodb    0 Oct 14 10:47 mongodb-27017.sock
drwx------. 3 root    root    4096 Nov 12 19:33 systemd-httpd.service-Xzx1e87

>> pulp-admin rpm repo export run --repo-id gena --export-dir /tmp/blabla
+----------------------------------------------------------------------+
                      Publishing Repository [gena]
+----------------------------------------------------------------------+

The following publish configuration options will be used:

Export Dir:  /tmp/blabla


This command may be exited via ctrl+c without affecting the request.


Initializing repo metadata
[-]
... completed

Publishing Distribution files
[-]
... completed

Publishing RPMs
[==================================================] 100%
2 of 2 items
... completed

Publishing Delta RPMs
... skipped

Publishing Errata
[-]
... completed

Publishing Comps file
[-]
... completed

Publishing Metadata.
[-]
... completed

Closing repo metadata
[-]
... completed

Generating sqlite files
... skipped

Copying files
[-]
... failed
[(u'/var/lib/pulp/working/repos/gena/distributors/export_distributor/repodata',
u'/tmp/blabla/gena/repodata', "[Errno 13] Permission denied:
'/tmp/blabla/gena/repodata'"),
(u'/var/lib/pulp/working/repos/gena/distributors/export_distributor',
u'/tmp/blabla/gena', "[Errno 13] Permission denied: '/tmp/blabla/gena'")]


Task Failed

[(u'/var/lib/pulp/working/repos/gena/distributors/export_distributor/repodata',
u'/tmp/blabla/gena/repodata', "[Errno 13] Permission denied:
'/tmp/blabla/gena/repodata'"),
(u'/var/lib/pulp/working/repos/gena/distributors/export_distributor',
u'/tmp/blabla/gena', "[Errno 13] Permission denied: '/tmp/blabla/gena'")]

>> ls -l /tmp
total 8
drwxr-xr-x. 3 apache  apache  4096 Nov 13 13:29 blabla
srwxrwxrwx. 1 mongodb mongodb    0 Oct 14 10:47 mongodb-27017.sock
drwx------. 3 root    root    4096 Nov 12 19:33 systemd-httpd.service-Xzx1e87

>> ls -l /tmp/blabla/
total 4
drwxr-xr-x. 3 apache apache 4096 Nov 13 13:29 gena


And now we export it to the existing directory:

>> mkdir /tmp/gena_export
>> chown apache:apache /tmp/gena_export/
>> ls -l /tmp
total 12
drwxr-xr-x. 3 apache  apache  4096 Nov 13 13:29 blabla
drwxr-xr-x. 2 apache  apache  4096 Nov 13 13:30 gena_export
srwxrwxrwx. 1 mongodb mongodb    0 Oct 14 10:47 mongodb-27017.sock
drwx------. 3 root    root    4096 Nov 12 19:33 systemd-httpd.service-Xzx1e87

>> pulp-admin rpm repo export run --repo-id gena --export-dir /tmp/gena_export
+----------------------------------------------------------------------+
                      Publishing Repository [gena]
+----------------------------------------------------------------------+

The following publish configuration options will be used:

Export Dir:  /tmp/gena_export


This command may be exited via ctrl+c without affecting the request.


Initializing repo metadata
[-]
... completed

Publishing Distribution files
[-]
... completed

Publishing RPMs
[==================================================] 100%
2 of 2 items
... completed

Publishing Delta RPMs
... skipped

Publishing Errata
[-]
... completed

Publishing Comps file
[-]
... completed

Publishing Metadata.
[-]
... completed

Closing repo metadata
[-]
... completed

Generating sqlite files
... skipped

Copying files
[-]
... failed
[Errno 13] Permission denied: '/tmp/gena_export/gena'


Task Failed

[Errno 13] Permission denied: '/tmp/gena_export/gena'

And ta-dam! It's empty!

>>ls -l /tmp/gena_export/
total 0