Split out the Package type into a first class Kubernetes resource (#295)

Split out the Package type into a first class Kubernetes resource. Before this change, packages were implicitly tied to functions.

This wasn't ideal because:
 * Functions will need to share packages
 * A package storage system may be more generally useful than just
   functions (for example, for storing static assets)

This change does the following:
* Updates the fission and tpr types to add a new Package and PackageSpec.  It also creates a PackageRef type, and a FunctionPackageRef type. The PackageRef simply references a package, but the FunctionPackageRef includes the name of a function within the package. This allows us to share packages between different functions.
* Updates fetcher and other components for first-class packages
* Allows customization of fetcher image pull policy in the helm charts

charts/fission-all/templates/deployment.yaml

@@ -238,6 +238,8 @@ spec:
         env:
         - name: FETCHER_IMAGE
           value: "{{ .Values.fetcherImage }}:{{ .Values.fetcherImageTag }}"
+        - name: FETCHER_IMAGE_PULL_POLICY
+          value: "{{ .Values.pullPolicy }}"
       serviceAccount: fission-svc
 
 ---

charts/fission-core/templates/deployment.yaml

@@ -238,6 +238,8 @@ spec:
         env:
         - name: FETCHER_IMAGE
           value: "{{ .Values.fetcherImage }}:{{ .Values.fetcherImageTag }}"
+        - name: FETCHER_IMAGE_PULL_POLICY
+          value: "{{ .Values.pullPolicy }}"
       serviceAccount: fission-svc
 
 ---

charts/fission-core/values.yaml

@@ -13,7 +13,7 @@ image: fission/fission-bundle
 imageTag: v0.2.0-20170822
 
 ## Image pull policy
-pullPolicy: ifNotPresent
+pullPolicy: IfNotPresent
 
 ## Fission fetcher repository
 fetcherImage: fission/fetcher