Signed-off-by: Petr Horacek <phoracek@redhat.com>

Use goimports to check code:
goimports checks what gofmt does plus it keeps imports in specific
order. In this patch we:
- replace gofmt with goimports
- add code format checking to travis
- reorganize Makefile a little bit
- remove make generate-manifests from README.md since it is not needed
  during the development process
- add Makefile target for operator-sdk build k8s

Add whitespace check and auto-formatting:
I hate trailing whitespaces. This patch adds automated check detecting
any and Makefile target removing them.

Add codegen validation:
Add a check validating that generated code is up to date. This would
fail on travis in case a contributor forget to run `make generate-api`.

ship and goimports operator-sdk in vendor/:
In order to make sure everyone uses the same version of operator-sdk
producing the same code, let's keep operator-sdk in vendoring. The
same for goimports.

Signed-off-by: Petr Horacek <phoracek@redhat.com>