Adding gitclonerssh plugin

Author: francisco-cocozza

plugins/gitclonerssh/README.md

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+# Clone repository via SSH
+Plugin to clone git repositories via SSH.
+
+Dockerhub repo: https://hub.docker.com/r/codefresh/cfstep-gitclonerssh
+
+## Options
+| ENVIRONMENT VARIABLE | DEFAULT | TYPE | REQUIRED | DESCRIPTION |
+|--|--|--|--|--|
+| REMOTE_URL | null | string | Yes | Reporitory SSH URL (e.g. `git@github.com:my-user/my-repo.git`) |
+| BRANCH | master | string | No | Branch name to checkout (e.g. `master`) |
+| SSH_KEY | null | string | Yes | Private SSH key to access the repository. To set a value for this var you can: `cat ~/.ssh/my_ssh_key_file | tr '\n' ','`. This assumes that `SPLIT_CHAR` will be set to `,` |
+| SPLIT_CHAR | null | string | Yes | Split character you’ve used to replace newline in SSH key (`SSH_KEY`). Recommendation: use `,` (comma character)|
+| CLONE_PATH | working directory | string | No | Path where `git clone` is going to be executed. A "`repo-name`" directory will be created there|
+
+## Usage Example:
+This  example clones a private repo using a private SSH key.
+This example assumes that `SSH_KEY` var is already saved as an encrypted-pipeline-var. And that `SPLIT_CHAR` has a value of ','.
+
+```yaml
+version: '1.0'
+steps:
+  clone_repo_via_ssh:
+    image: codefresh/cfstep-gitclonerssh
+    environment:
+      - REMOTE_URL=git@github.com:my-user/my-repo.git
+      - BRANCH=my-branch
+      - SSH_KEY=${{SSH_KEY}}
+      - SPLIT_CHAR=${{SPLIT_CHAR}}
+      - CLONE_PATH=/codefresh/volume 
+```

plugins/gitclonerssh/plugin.yaml

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+image: codefresh/cfstep-gitclonerssh
+tag: latest
+version: 0.1.0
+description: clone git repositories via SSH.
+keywords:
+  - git
+  - clone
+  - ssh
+home: https://github.com/codefresh-io/cfstep-gitclonerssh
+sources:
+  - https://github.com/codefresh-io/cfstep-gitclonerssh
+maintainers:
+  - name: Francisco Cocozza
+    email: francisco@codefresh.io
+icon: https://git-scm.com/images/logos/downloads/Git-Icon-1788C.png
+  envs:
+    - name: REMOTE_URL
+      type: required
+      description: Reporitory SSH URL (e.g. `git@github.com:my-user/my-repo.git`)
+    - name: BRANCH
+      type: optional
+      description: Branch name to checkout (e.g. `master`)
+    - name: SSH_KEY
+      type: required
+      description: Private SSH key to access the repository.
+    - name: SPLIT_CHAR
+      type: required
+      description: Split character you’ve used to replace newline in SSH key
+    - name: CLONE_PATH
+      type: optional
+      description: Path where `git clone` is going to be executed. A "`repo-name`" directory will be created there