Add ENV var to disable MWC init

[phenixblue]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation

/kind feature

/kind release

What this PR does / why we need it:

This adds a new IMAGESWAP_DISABLE_AUTO_MWC environment variable to allow for disabling the automatic generation of the Mutating Webhook Configuration (MWC) in the imageswap-init container.

This allows for the MWC to be generated outside of the imageswap init process and adds more flexibility to integrate cleanly with other provisioning tools/flows.

Which issue(s) this PR fixes:

Fixes #67

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

- Adds the `IMAGESWAP_DISABLE_AUTO_MWC` environment variable to allow for disabling the automatic generation of the Mutating Webhook Configuration (MWC) in the imageswap-init container.

Additional documentation e.g., usage docs, etc.:

[phenixblue]

Just discovered that several of the existing Python unit tests were exhibiting missing key errors due to an already merged update to the structure of the admission response in the admission.k8s.io/v1 API. The field patchtype was changed to patchType (note the capital T). Fixed these tests as part of this PR.

[ilrudie]

looks good to me