Add environment variable for all flags and envvar prefix

[albertrdixon]

What

• Add environment variable for provider flag
• Add environment variable for google-project flag
• Add optional environment variables format with SECRETS_INIT prefix

Why

• Using secrets init for apps in GCP required setting the command line flag rather than the much easier (in kubernetes at least) environment variable flow
• Using a prefix for environment variables helps to avoid collision (in particular with LOG_FORMAT)

[sakomws]

hey, @alexei-led help is appreciated with this PR review pls.

[alexei-led]

Thank you for this PR. Please make sure all ENV variables are consistent: two ENV vars per parameter (one with prefix and another without)