Sort requirement files in alphabetical order

This makes code more readable, and can check whether specific library
in the requirement files easily. We also enforce the check in pep8.

Change-Id: I08434dd56d41a82045e4119e547b7b3f60d66698
Closes-Bug: #1285478
Python 3: Fix YamlEnvironmentTest tests

In Python 3, this happens:

>>> 'foo' in ValueError('foo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: argument of type 'ValueError' is not iterable

We have to cast the Exception to make the tests work.

Change-Id: Ifc06c9860412030dc697b264812a6bf78e10eb8b
Using common methods from oslo cliutils

Synchronize the cliutils from oslo to heatclient and use
the common methods.

used common methods:
+arg
+env
+print_list

partially implements blueprint common-client-library-2

Change-Id: If79d58794257325a47b9938a172a87097fc93e1b
Merge "Python 3: Fix YamlEnvironmentTest tests"
Merge "Using common methods from oslo cliutils"
Merge "Sort requirement files in alphabetical order"