Add feature flag support

[kragniz]

What this PR does / why we need it: adds support for generic feature flags

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #335

Special notes for your reviewer:

Release note:

Add support for feature flags

[jetstack-bot]

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[wallrj]

The output looks like this:

./navigator-controller_linux_amd64 --help

cert-manager is a Kubernetes addon to automate the management and issuance of
TLS certificates from various issuing sources.

It will ensure certificates are valid and up to date periodically, and attempt
to renew certificates at an appropriate time before expiry.

Usage:
  cert-manager-controller [flags]

Flags:
      --alsologtostderr                           log to standard error as well as files
      --feature-gates string                      A set of key=value pairs that describe feature gates for various features. Options are:
Example=true|false (ALPHA - default=false)
...

[jetstack-bot]

@kragniz: PR needs rebase.

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