Allow disabling resource limits by default in agent injector

[mprasil]

Is your feature request related to a problem? Please describe.

#174 allows to not add resource limits to the injected container by providing empty string as value in relevant annotation. However when trying to set this as default via environment variable (for example using AGENT_INJECT_CPU_LIMIT) this does not work as environment variable with empty string value is ignored and default is used instead.

Describe the solution you'd like

Either empty string is accepted as value and then same mechanism will apply as for annotation with empty string or some other value like none is used for the same purpose.

Describe alternatives you've considered

This is primary mechanism the vault helm chat is using to set the default values, so there aren't really any alternatives other than adding the vault.hashicorp.com/agent-limits-cpu: "" annotation to every deployment, which is quite cumbersome.