Add retrieve to compilation spec

It allows to copy build artifact during buildtime. In this way
artifacts between different seed images can be shared

```
retrieve:
- a-test-1.0.package.*
- https://...
```

They will be available under WORKDIR

tests/fixtures/retrieve-integration/a/build.yaml

@@ -0,0 +1,3 @@
+image: "alpine"
+steps:
+  - echo a > /a

tests/fixtures/retrieve-integration/a/definition.yaml

@@ -0,0 +1,3 @@
+name: "a"
+version: "1.0"
+category: "test"

tests/fixtures/retrieve-integration/b/build.yaml

@@ -0,0 +1,10 @@
+
+steps:
+- tar xvf a-test-1.0.package.* -C ./
+- mv a /b
+requires:
+- name: "a"
+  version: "1.0"
+  category: "test"
+retrieve:
+- a-test-1.0.package.*

tests/fixtures/retrieve-integration/b/definition.yaml

@@ -0,0 +1,7 @@
+name: "b"
+version: "1.0"
+category: "test"
+requires:
+- name: "a"
+  version: "1.0"
+  category: "test"

tests/fixtures/retrieve/a/build.yaml

@@ -0,0 +1,10 @@
+image: "luet/base"
+seed: "alpine"
+steps:
+  - echo foo > /test
+  - echo bar > /test2
+retrieve:
+- test
+- http://www.google.com
+env:
+- test=1

tests/fixtures/retrieve/a/definition.yaml

@@ -0,0 +1,3 @@
+name: "a"
+version: "1.0"
+category: "test"

tests/integration/04_retrieve.sh

@@ -0,0 +1,90 @@
+#!/bin/bash
+
+export LUET_NOLOCK=true
+
+oneTimeSetUp() {
+export tmpdir="$(mktemp -d)"
+}
+
+oneTimeTearDown() {
+    rm -rf "$tmpdir"
+}
+
+testBuild() {
+    mkdir $tmpdir/testbuild
+    luet build --tree "$ROOT_DIR/tests/fixtures/retrieve-integration" --destination $tmpdir/testbuild --compression gzip test/b
+    buildst=$?
+    assertEquals 'builds successfully' "$buildst" "0"
+    assertTrue 'create package dep B' "[ -e '$tmpdir/testbuild/b-test-1.0.package.tar.gz' ]"
+    assertTrue 'create package' "[ -e '$tmpdir/testbuild/a-test-1.0.package.tar.gz' ]"
+}
+
+testRepo() {
+    assertTrue 'no repository' "[ ! -e '$tmpdir/testbuild/repository.yaml' ]"
+    luet create-repo --tree "$ROOT_DIR/tests/fixtures/retrieve-integration" \
+    --output $tmpdir/testbuild \
+    --packages $tmpdir/testbuild \
+    --name "test" \
+    --descr "Test Repo" \
+    --urls $tmpdir/testrootfs \
+    --type disk > /dev/null
+
+    createst=$?
+    assertEquals 'create repo successfully' "$createst" "0"
+    assertTrue 'create repository' "[ -e '$tmpdir/testbuild/repository.yaml' ]"
+}
+
+testConfig() {
+    mkdir $tmpdir/testrootfs
+    cat <<EOF > $tmpdir/luet.yaml
+general:
+  debug: true
+system:
+  rootfs: $tmpdir/testrootfs
+  database_path: "/"
+  database_engine: "boltdb"
+repositories:
+   - name: "main"
+     type: "disk"
+     enable: true
+     urls:
+       - "$tmpdir/testbuild"
+EOF
+    luet config --config $tmpdir/luet.yaml
+    res=$?
+    assertEquals 'config test successfully' "$res" "0"
+}
+
+
+
+testInstall() {
+    luet install --config $tmpdir/luet.yaml test/b
+    #luet install --config $tmpdir/luet.yaml test/c-1.0 > /dev/null
+    installst=$?
+    assertEquals 'install test successfully' "$installst" "0"
+    assertTrue 'package B installed' "[ -e '$tmpdir/testrootfs/b' ]"
+    val=$(cat "$tmpdir/testrootfs/b")
+    assertEquals 'package B content comes from a' "$val" "a"
+    assertTrue 'package A installed' "[ -e '$tmpdir/testrootfs/a' ]"
+}
+
+
+testUnInstall() {
+    luet uninstall --config $tmpdir/luet.yaml test/b
+    installst=$?
+    assertEquals 'uninstall test successfully' "$installst" "0"
+    assertTrue 'package uninstalled' "[ ! -e '$tmpdir/testrootfs/b' ]"
+    assertTrue 'package uninstalled' "[ ! -e '$tmpdir/testrootfs/a' ]"
+}
+
+
+testCleanup() {
+    luet cleanup --config $tmpdir/luet.yaml
+    installst=$?
+    assertEquals 'install test successfully' "$installst" "0"
+    assertTrue 'package installed' "[ ! -e '$tmpdir/testrootfs/packages/b-test-1.0.package.tar.gz' ]"
+}
+
+# Load shUnit2.
+. "$ROOT_DIR/tests/integration/shunit2"/shunit2
+