ctrcfg -- New content page for API 'config'

Centralized Persistent Configuration

With the exception of Sync Gateway replications (ISGR), the only way to persist configuration settings or updates is through updates to the Sync Gateway JSON config file, typically requiring a restart of the Sync Gateway in order to take effect.

Provide a centralized peristent config for iSGR

PRD: https://docs.google.com/document/d/1tGtc1fQt0aK1QqzYKyeQTC7UpR-O-sfISgkC4MHdNUU/edit?usp=sharing

Include planned changes to: 

• Cover use of API to make persistent changes
• Include any scenario differences an/or constraints (eg ISGR vs SGW sync)
• Document any behavior changes
• Summarize behavior changes in release notes as necessary

(extract) ... The “cluster-aware” configuration enhancements will vastly simplify the administration and management of Sync Gateway clusters by supporting dynamic, modular and persistent config changes via a RESTful interface .

https://docs.google.com/document/d/1tGtc1fQt0aK1QqzYKyeQTC7UpR-O-sfISgkC4MHdNUU/edit#

• The size of Sync Gateway clusters are growing and many deployments are homogeneous in configuration. Requiring the same config change to be made on each sync gateway node individually is tedious. 
• The ability to dynamically update the configuration of a Sync Gateway cluster in a persistent fashion would ensure that changes are not lost during sync gateway restarts (crashes or deliberate restarts) potentially leading to inconsistent system state.
• The ability to remotely administer and update the run time configuration of sync gateway config via REST endpoint would vastly simplify manageability in a managed cloud environment (such as Couchbase Cloud). 
• Sync Gateway cluster in multi-tenant applications handle databases for different tenants. Any configuration change made for one tenant or adding a new tenant to the cluster would require a restart of the cluster impacting all tenants which is undesirable. This would be more prevalent with collections support in future.
• The current monolithic config file is difficult to maintain, inflexible and subject to user error. Modularizing the configuration  would simplify maintenance.