Standby create replica methods

[eberlep]

• Add additional create_replica_method
• Add STANDBY_* variables

In theory, this should allow us to simply bootstrap a standby via S3:

patronictl remove <clustername>

env:
- name: STANDBY_USE_WALG_BACKUP
  value: "true"
- name: STANDBY_USE_WALG_RESTORE
  value: "true"
- name: STANDBY_WITH_WALG
  value: "true"
- name: STANDBY_AWS_ACCESS_KEY
  value: "BAR"
- name: STANDBY_AWS_SECRET_ACCESS_KEY
  value: "FOO"
- name: STANDBY_AWS_REGION
  value: "us-east-1"
- name: STANDBY_AWS_ENDPOINT
  value: "https://foobar"
- name: STANDBY_AWS_WALG_S3_ENDPOINT
  value: "https+path://foobar"
- name: STANDBY_AWS_S3_FORCE_PATH_STYLE
  value: "true"
- name: STANDBY_AWS_REGION
  value: "us-east-1"
- name: STANDBY_WALG_DISABLE_S3_SSE
  value: "true"
- name: STANDBY_WALG_S3_SSE
  value: ""
- name: STANDBY_WALG_S3_PREFIX
  value: "s3://foo/bar"
- name: STANDBY_WALG_S3_ENDPOINT
  value: "https+path://foobar"