Add default ImagePullSecret if not present

[ferenc-hechler]

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:

As the images are rewritten to be pulled from another Docker Registry,
it might also be neccessary to add the corresponding Image-Pull-Secret
for authentication.

Which issue(s) this PR fixes:

Fixes #123

Special notes for your reviewer:

As the project looks quite inactive, I did not take care about tests and documentation.
If the PR is actively reviewed, I will add this.

Does this PR introduce a user-facing change?:

NONE

Allow injecting a default ImagePullSecret to every POD.

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

This feature is only active, if the environment variable $IMAGESWAP_DEFAULT_IMAGE_PULL_SECRET is set.

As Kubernetes has a special behavior whether there exist at least one imagePullSecrets in a POD or not, 
there is a possibility to suppress the injection with $IMAGESWAP_ALWAYS_ADD_DEFAULT_IPS="false".
If a POD has no ImagePullSecrets, then the Secrets are taken from the ServiceAccount.