Implement delete entity mixin

[abalakh]

Similar to robottelo.ui.Base.delete we should create a helper method which will handle entity deletion and ensure deleted entity can't be found using search.
Couple of improvements which would be nice to have (thanks @oshtaier for pointing them):

• cover the case when deleted entity was the last one available, so searchbar is not present anymore on entity list page
• delete should return True/False whether deletion succeeded so we can easily assert deletion in tests like:

assert session.architecture.delete('foo')

so we could mark the test as failed (not errored) in case element was found.

[mfalesni]

I think using exceptions may be more beneficial, you just do:

with some_expected_error_thing(FlashMessageException):
    create_bad_entity()

If a different error happens, it throws a different exception and that can be categorized.