greeter randomly ignores selected session

# Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
The greeter randomly ignores the selected session (e.g. pantheon, kde, etc.).

I can choose between the following sessions:
- pantheon
- steamos

### To Reproduce
Assuming you have an additional DE (unity, gnome, kde, steamos, etc.) installed

Steps to reproduce the behaviour:
1. Go to greeter
2. Click on the "gearwheel" to the right side of the password input field
3. Choose "pantheon"
4. Type in you password
5. Login
6. See the DE you logged in the last time (e.g. steamos)

### Expected behaviour

See the expected session. According to the previous example, I would expect to see a pantheon session.

## Platform Information

Please check what applies:
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- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version