Pin build.lock for all layers to the commit hash

The build.lock branch points to master which means that the layer is not
locked.  Lock the layers to the commit hash instead; this ensures
reproducible builds as much as possible.

Change-Id: I0b74fe209741c0ba86f3d903056a47ed702ef169

src/build.lock

@@ -5,7 +5,7 @@
       "item": "layer:options",
       "url": "https://github.com/juju-solutions/layer-options.git",
       "vcs": null,
-      "branch": "refs/heads/master",
+      "branch": "fcdcea4e5de3e1556c24e6704607862d0ba00a56",
       "commit": "fcdcea4e5de3e1556c24e6704607862d0ba00a56"
     },
     {
@@ -13,7 +13,7 @@
       "item": "layer:basic",
       "url": "https://github.com/juju-solutions/layer-basic.git",
       "vcs": null,
-      "branch": "refs/heads/master",
+      "branch": "a3ff62c32c993d80417f6e093e3ef95e42f62083",
       "commit": "a3ff62c32c993d80417f6e093e3ef95e42f62083"
     },
     {
@@ -21,7 +21,7 @@
       "item": "layer:openstack",
       "url": "https://github.com/openstack/charm-layer-openstack",
       "vcs": null,
-      "branch": "refs/heads/master",
+      "branch": "207bac8bec837edc6a4d6003f51d5b59d2d75adb",
       "commit": "207bac8bec837edc6a4d6003f51d5b59d2d75adb"
     },
     {
@@ -37,7 +37,7 @@
       "item": "interface:tls-certificates",
       "url": "https://github.com/juju-solutions/interface-tls-certificates",
       "vcs": null,
-      "branch": "refs/heads/master",
+      "branch": "d9850016d930a6d507b9fd45e2598d327922b140",
       "commit": "d9850016d930a6d507b9fd45e2598d327922b140"
     },
     {
@@ -45,7 +45,7 @@
       "item": "interface:cinder-backup",
       "url": "https://github.com/openstack/charm-interface-cinder-backup.git",
       "vcs": null,
-      "branch": "refs/heads/master",
+      "branch": "a1cdf71ad61d4ea5f912d8e0423e887ae761bff7",
       "commit": "a1cdf71ad61d4ea5f912d8e0423e887ae761bff7"
     },
     {