Added oslo.context to requirements.txt

Added missing dependency oslo.context to requirements.txt

Change-Id: I88c42fd2381bad55ff499e096a93dcc2cc1d44e5
Closes-Bug: #1560976

requirements.txt

@@ -7,6 +7,7 @@ jsonpatch>=1.1 # BSD
 keystoneauth1>=2.1.0 # Apache-2.0
 keystonemiddleware!=4.1.0,>=4.0.0 # Apache-2.0
 oslo.config>=3.7.0 # Apache-2.0
+oslo.context>=0.2.0 # Apache-2.0
 oslo.db>=4.1.0 # Apache-2.0
 oslo.i18n>=2.1.0 # Apache-2.0
 oslo.log>=1.14.0 # Apache-2.0