Remove iso8601 dependency

Murano does not import and use this module directly,
no need to list it in the requirements.

Change-Id: Ia157fa7e6d692946a2d9ab47d65dab5bdd2dca89

requirements.txt

@@ -14,7 +14,6 @@ retrying!=1.3.0,>=1.2.3 # Apache-2.0
 WebOb>=1.2.3
 kombu>=3.0.7
 psutil<2.0.0,>=1.1.1
-iso8601>=0.1.9
 six>=1.9.0
 netaddr!=0.7.16,>=0.7.12
 PyYAML>=3.1.0