Allow override Redis host/port/prefix via defines

[vincent-lu]

The goal is to allow defining Redis cache's host/port/prefix without using nginx helper's settings backend page. The three defines are:

RT_WP_NGINX_HELPER_REDIS_HOSTNAME
RT_WP_NGINX_HELPER_REDIS_PORT
RT_WP_NGINX_HELPER_REDIS_PREFIX

If not defined, everything works as is (fallback to the settings stored in options, if empty, fallback to 127.0.0.1/6379/nginx-cache:

If defined, the input fields for the predefined redis settings are disabled, with note underneath to inform the user this field is overridden by defines.

Documentation (FAQ in README.md) is updated to reflect this feature.

The main benefit of this is in multiple server situations (production vs staging, load balanced servers, etc) you don't have to rely on database to inform nginx helper about where to find the redis server.

Thanks and please consider this PR.

[vincent-lu]

Ping @chandrapatel , thanks for the great work, again.

[mbtamuli]

Testing this PR.

[vincent-lu]

Thanks @mbtamuli let me know if you have any suggestions for this PR.

[mbtamuli]

Tested this PR. This changes made in this PR works perfectly as intended.