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: I820e01002a798bdcf198a1b14db8411c6340b278

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:juju-info",
       "url": "https://github.com/juju-solutions/interface-juju-info.git",
       "vcs": null,
-      "branch": "refs/heads/master",
+      "branch": "4e2f90052b3c02031d09f10900c9e9cb22565dee",
       "commit": "4e2f90052b3c02031d09f10900c9e9cb22565dee"
     },
     {
@@ -53,7 +53,7 @@
       "item": "interface:vault-kv",
       "url": "https://github.com/openstack-charmers/charm-interface-vault-kv.git",
       "vcs": null,
-      "branch": "refs/heads/master",
+      "branch": "dceab99ac3739cc7265e386287f100f1bfebc47f",
       "commit": "dceab99ac3739cc7265e386287f100f1bfebc47f"
     },
     {
@@ -61,7 +61,7 @@
       "item": "interface:barbican-secrets",
       "url": "https://github.com/openstack/charm-interface-barbican-secrets.git",
       "vcs": null,
-      "branch": "refs/heads/master",
+      "branch": "1be4ff07766e51523be56115bb7df5e0cd82055f",
       "commit": "1be4ff07766e51523be56115bb7df5e0cd82055f"
     },
     {