Update ChangeLog and version files for v0.21

ChangeLog

+2016-11-17, v0.21
+	* Add a new auth_keys OS parameter for injecting SSH keys to the
+	  instance during creation
+	* Add DEFAULT_NIC_CONFIG configuration parameter for configuring the
+	  default NIC
+	* Fix small bugs
+
 2016-11-10, v0.20.2
 	* Fix a bug where the disk was truncated to the size of the image file
 	  if the used disk template was "file" or "sharedfile"

docs/version.py

-__version__ = "0.21.dev0"
+__version__ = "0.21"

version.m4

-m4_define([devflow_version], [0.21.dev0])
+m4_define([devflow_version], [0.21])