Add Gentoo support to growroot

Add missing growroot initscript and pkg-map entries for Gentoo.

growpart was added to Gentoo with [1]

Update the readme to reflect reality too (fedora added with
I5630dc638f85b1e80795826ef36a306632075460)

[1] https://packages.gentoo.org/packages/sys-fs/growpart

Closes-Bug: #1539273
Change-Id: I29056c7297489ec04f37757dbe33976901eceb49

elements/growroot/README.rst

@@ -4,4 +4,8 @@ growroot
 
 Grow the root partition on first boot.
 
-This only supported on ubuntu trusty or later.
+This is only supported on:
+
+* ubuntu trusty or later
+* gentoo
+* fedora & centos

elements/growroot/init-scripts/openrc/growroot

@@ -0,0 +1,10 @@
+#!/sbin/runscript
+
+start() {
+    /usr/local/sbin/growroot
+    eend 0
+}
+
+depend() {
+    need localmount
+}

elements/growroot/pkg-map

@@ -7,6 +7,10 @@
         "debian":{
             "growpart": "cloud-utils",
             "e2fsprogs": "e2fsprogs"
+        },
+        "gentoo": {
+            "growpart": "sys-fs/growpart",
+            "e2fsprogs": "sys-fs/e2fsprogs"
         }
     }
 }