Convert Cert Manager app to new style

[yankeexe]

Use DefaultInstallOptions pattern for cert-manager installation.

Fixes #225

Signed-off-by: yankeexe yankee.exe@gmail.com

Description

Based on the new pattern of app installation on apps packages,
installation of cert-manager has been changed to utilize the
DefaultInstallOptions method.

Motivation and Context

• I have raised an issue to propose this change (required)
  Fixes Convert Cert Manager app to new style #225

How Has This Been Tested?

Created a local kind cluster and installed cert-manager.

$ kind create cluster 

Scenario 1

Default Install with no options.

$ ./arkade install cert-manager 

Scenario 2
Install cert-manager in custom namespace.

$ ./arkade install cert-manager -n "cert-test"

Scenario 3
Install user-defined version of cert-manager.

$ ./arkade install cert-manager -v "v0.15.1"

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

• I have tested this on arm, or have added code to prevent deployment

[Waterdrips]

LGTM

[alexellis]

LGTM