Add installer tests
Improve documentation for robottelo.log module
Improve installer logfile check test
Merge pull request #864 from elyezer/installer-tests

Add installer tests
Refactor decorators

Split method bzbug into bzbug and get_bugzilla_bug. Add method
skip_if_bz_bug_open. get_bugzilla_bug is reponsible for fetching and caching
bug objects, bzbug can be used as usual, and skip_if_bz_bug_open causes the
currently running test to abort if the named bug is open.

The exact conditions under which tests will be skipped have changed. For
example, formerly, a test would skip if the relevant bug could not be fetched.
Now, the test will run if the relevant bug cannot be fetched.
Use skip_if_bz_bug_open instead of bzbug

Decorators are executed when a function is defined. As a result, importing a
module containing containing functions decorated with `bzbug` is an extremely
slow process: each of those funciton definitions triggers communications with a
Bugzilla server. The `skip_if_bz_bug_open` function accomplishes the same goal
of skipping certain tests. However, it does not incur communication with a
Bugzilla server until the test function is actually run.
Remove unused function `bzbug`
Remove unused method `is_bzbug`

Also remove now-unnecessary imports.
Move method to different module. Fix imports.

Move method `skip_if_bz_bug_open` from module `robottelo.common.decorators` to
`robottelo.common.helpers`. Do this b/c the method is not a decorator. Fix
imports to accomodate this change.