Add dcos library

The project kolla-mesos needs a dcos library for communication
with Mesos. This is an essential requirement for it.

Is the library actively maintained?
===================================

Yes, it's an active project on Github [1] with the latest release
from 19th Oct. [2]

Is the library good code?
=========================

It's mostly covered by unit tests and PEP8 compatible.

Is the library python 3 compatible?
===================================

Yes, it uses six library for compatibility with Py 3.x. Both
Py 2.7 and 3.4 are tested.

Is the library license compatible?
==================================

Yes, it's licensed under Apache License 2.0.

Is the library already packaged in the distros we target?
=========================================================

No, but packaging it would be simple. Furthermore, this library
will be used only in kolla-mesos project, which is in very early
development phase and will have to be packaged too in future.

Is the function of this library already covered by other libraries?
===================================================================

No, there is no library interacting with Mesos in global requirements.

[1] https://github.com/mesosphere/dcos-cli
[2] https://github.com/mesosphere/dcos-cli/releases/tag/0.2.0

Change-Id: Idf4ac1c714094d91885f0918dc276b21d73e9e3b

global-requirements.txt

@@ -22,6 +22,7 @@ PuLP>=1.0.4
 contextlib2>=0.4.0 # PSF License
 croniter>=0.3.4 # MIT License
 cryptography>=1.0 # Apache-2.0
+dcos>=0.1.3 # Apache-2.0
 ddt>=0.7.0
 debtcollector>=0.3.0  # Apache-2.0
 decorator>=3.4.0

upper-constraints.txt

@@ -88,6 +88,7 @@ contextlib2===0.4.0
 coverage===4.0.3
 croniter===0.3.9
 cryptography===1.1.1
+dcos===0.1.3
 ddt===1.0.1
 debtcollector===1.0.0
 decorator===4.0.4