Make string formatting work on RHEL6 python 2.6

$ python
Python 2.6.6 (r266:84292, Nov 21 2013, 10:50:32)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> "{} {}".format('hello', 'world')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: zero length field name in format
>>> "{0} {1}".format('hello', 'world')
'hello world'
Fixing line lenghts
Another attempt to fix lint issues
Merge pull request #902 from jhutar/master

Make string formatting work on RHEL6 python 2.6
Update config properties to be a defaultdict

All asked keys for the config will return None if the properties file is
not available or the key is not present. This will make docs build but
will need to think better on the framework configuration in the future.
Further refactor robottelo/common/__init__.py

Prevent class `Config` from returning a default value of `None` when its
instance var `properties` is read. Instead, make `properties` an normal dict.
If calling code reads a nonexistent value from `conf.properties`, (and this
will happen for all reads if no config file is found) that calling code now
receives a KeyError exception.

It may be useful to set some default values in the configuration object. For
example, a default locale may be appropriate.
Merge pull request #903 from Ichimonji10/update-config

Update config
Make travis run tests in `tests/robottelo/`

The test suite will fail unless the packages listed in `requirements.txt` are
installed. Make Travis install them.

Also add a comment explaining why the tests in `tests/foreman/` are not run.
Remove unnecessary static vars

This fix allows the relevant class to be imported even when no config file is
present, and therefore fixes one of the errors issued when running unit tests
in `tests/robottelo/`. Also, it's kinda silly to add static vars to a class for
no good reason.
Fix test in module tests.robottelo.test_ssh

* Fix all references to the instance variable `Config._properties`.
* Populate the config file dictionary with test values.
* Create a DSA private key for test purposes
Make API docs reference recently added module
Reorder API documentation content

If a package contains any sub-packages, links to those sub-packages should
appear at the very top of the relevant documentation. This is in line with user
expectations: navigation elements typically appear above content.
Fix a bug in module robottelo.common.ssh
Merge pull request #906 from Ichimonji10/ssh-bugfix

Fix a bug in module robottelo.common.ssh
Merge pull request #904 from Ichimonji10/robo-tests

make Travis run more tests