User is prompted to select a space instead of dashboard after loggin in on an iFrame

[bhavyarm]

Kibana version: 6.5.0 BC6

Elasticsearch version: 6.5.0 BC6

Browser version: chrome latest

Original install method (e.g. download page, yum, from source, etc.): from staging

Describe the bug: If a role as access permissions to multiple spaces - user with that role is given the option of selecting a space on an iFrame when she logs in instead of being redirected to the shared object in iFrame.

Steps to reproduce:

1. create 2 spaces space z and space y along with default space
2. Create a spaceyreader role - give it read and view_index_metadata privilege on logstash_* index pattern - read privileges space z and all privileges to space y
3. Create an spaceyreader user and assign spaceyreader role to it
4. Create a dashboard with objects from logstash - and create an iFrame out of it.
5. Logout of main Kibana - open the html with iFrame and try to login with the user spaceyreader
6. User is presented with an option of selecting a space z or space y instead of being redirected to dashboard
7. Once user clicks on a space - she is redirected to Kibana home instead of the dashboard

Expected behavior: User should be shown the dashboard instead of space selector

[elasticmachine]

Pinging @elastic/kibana-security