Robottelo: Solve timetout issues when manipulating manifest

[renzon]

Tests which manipulates manifest take too long when satellite is full of data used on prior tests

See #3618 conversation for reference about the issue.

3617 is also affected by this

[renzon]

@rochacbruno give an idea of running this tests before all the others. Maybe tear down can also solve this.

[renzon]

@omaciel @sghai I closed the related issues in favor of this one. Waiting for your ideas.

[sghai]

commented here: https://github.com/SatelliteQE/robottelo/issues/3618#issuecomment-233839221

[rplevka]

@omaciel I'd like to propose closing this issue in favor of: https://github.com/SatelliteQE/robottelo/issues/4028