Reana-job-controller

--------------------------

- Adds support for specifying ``htcondor_max_runtime`` and ``htcondor_accounting_group`` for HTCondor compute backend jobs.
- Fixes Docker build by properly exiting when there are problems with ``myschedd`` installation.

--------------------------

- Adds support for running unpacked Docker images from CVMFS on HTCondor jobs.
- Adds support for pulling private images using image pull secrets.
- Adds support for VOMS proxy as a new authentication method.
- Adds pinning of all Python dependencies allowing to easily rebuild component images at later times.
- Fixes HTCondor job submission retry technique.
- Changes error reporting on Docker image related failures.
- Changes runtime pods to prefix user workflows with the configured REANA prefix.
- Changes CVMFS to be read-only mount.
- Changes runtime job instantiation into the configured runtime namespace.
- Changes test suite to enable running tests locally also on macOS platform.
- Changes CERN HTCondor compute backend to use the new ``myschedd`` connection library.
- Changes CERN Slurm compute backend to improve job status detection.
- Changes base image to use Python 3.8.
- Changes code formatting to respect ``black`` coding style.
- Changes documentation to single-page layout.

--------------------------

- Upgrades REANA-Commons package using latest Kubernetes Python client version.

--------------------------

- Adds generic job manager class and provides example classes for CERN HTCondor
and CERN Slurm clusters.
- Moves job controller to the same Kubernetes pod with the
REANA-Workflow-Engine-* (sidecar pattern).
- Adds sidecar container to the Kubernetes job pod if Kerberos authentication
is required.
- Provides user secrets to the job container runtime tasks.
- Refactors job monitoring using singleton pattern.

--------------------------

- Pins ``urllib3`` due to a conflict while installing ``Kubernetes`` Python
library.
- Fixes documenation build badge.

--------------------------

- Adds a new endpoint to delete jobs (Kubernetes).
- Introduces new common interface for job management which defines what the
compute backends should offer to be compatible with REANA, currently only
Kubernetes backend is supported.
- Fixes security vulnerability which allowed users to access other people's
workspaces.
- Makes CVMFS mounts optional and configurable at repository level.
- Updates the creation of CVMFS volumes specification, it now uses normal
persistent volume claims.
- Increases stability and improves test coverage.