Add secrets generator phase

This phase builded on top of generic executor container.
It uses kustomize generator to generate secrets
and SOPS function to encrypt secrets.

Usage:
    1. `curl -fsSL -o key.asc https://raw.githubusercontent.com/mozilla/sops/master/pgp/sops_functional_tests_key.asc`
       Copy existing key from sops project
    2. `export SOPS_IMPORT_PGP="$(cat key.asc)" && export SOPS_PGP_FP="FBC7B9E2A4F9289AC0C1D4843D16CEE4A27381B4"`
    3. `airshipctl phase run secret-generate`
        It will generate and encrypt secret in
        manifests/site/test-site/target/generator/results/generated/
    4. `KUSTOMIZE_PLUGIN_HOME=$(pwd)/manifests SOPS_IMPORT_PGP=$(cat key.asc) kustomize build --enable_alpha_plugins
        manifests/site/test-site/target/catalogues/ > output.txt`
	It will decrypt encrypted secret

Co-authored-by: Alexey Odinokov <aodinokov@mirantis.com>
Change-Id: I1682d71b7805eb36c407e712dcb747de799bc8bb
Relates-To: #379

playbooks/get-vm-config.yaml

@@ -21,6 +21,7 @@
   environment:
     KUSTOMIZE_PLUGIN_HOME: "/tmp"
     KUSTOMIZE_ENABLE_ALPHA_COMMANDS: "true"
+    SOPS_IMPORT_PGP: "{{ airship_config_pgp }}"
 
 - set_fact:
     bmh: "{{ bmh_command.stdout | from_yaml_all | list }}"
@@ -36,6 +37,7 @@
   environment:
     KUSTOMIZE_PLUGIN_HOME: "/tmp"
     KUSTOMIZE_ENABLE_ALPHA_COMMANDS: "true"
+    SOPS_IMPORT_PGP: "{{ airship_config_pgp }}"
   with_items: "{{ bmh }}"
 
 - name: get links from network data per BareMetalHost object