Support env vars in provider

[mzwennes]

In CICD pipelines it can be useful to inject certain values such as secrets or endpoints (when using multiple environments). This change allows a user to set an environment variable instead of hard-coding the api_token and endpoint in the provider block.

How to test

export INSTANA_API_TOKEN="<TOKEN>"
export INSTANA_ENDPOINT="<ENDPOINT_URL>"
terraform plan

Provider block can be empty when the above two env vars are set:

provider "instana" {}

[gessnerfl]

Much appreciate this change. Again thanks for your contribution @mzwennes