Add selectively pip install requirements

requirements/common.txt

@@ -2,12 +2,10 @@ pbr>=0.5.21,<1.0
 
 Babel>=1.3
 netaddr>=0.7.6
-falcon>=0.1.6,<0.1.7
 jsonschema>=1.3.0,!=1.4.0
 iso8601>=0.1.8
 msgpack-python
-pymongo>=2.4
-cassandra-driver>=1.0.0
+
 python-keystoneclient>=0.4.1
 python-memcached
 WebOb>=1.2.3,<1.3

requirements/requirements.txt

@@ -0,0 +1,4 @@
+-r common.txt
+-r storage/cassandra.txt
+-r transport/falcon.txt
+-r provider/fastly.txt

requirements/storage/cassandra.txt

@@ -0,0 +1 @@
+cassandra-driver>=1.0.0

requirements/storage/mongo.txt

@@ -0,0 +1 @@
+pymongo>=2.4

requirements/transport/falcon.txt

@@ -0,0 +1 @@
+falcon>=0.1.6,<0.1.7