[redis] allow configuration via REDIS_URL

[mikz]

closes #301

Environment variable REDIS_URL can be set to following format:
redis://PASSWORD@HOST:PORT/DB.

This enables connecting to password protected Redis on different DB than 0.

[octobot]

Proselint found issues

doc/parameters.md

Line	Message	Severity

Spell Checker found issues

doc/parameters.md

Line	Typo
3	APIcast v2 has a number of parameters co
3	ariables) that can modify the behavior of the gateway. The following
5	e that when deploying APIcast v2 with OpenShift, some of thee
15	Default: **stderr**
17	or more information. The file pathcan be either absolute, or relati
21	nfo
21	warn
37	Default: "apicast"
68	Values: a number > **60**
69	Default: 0
71	r. The value should be set to 0 or more than 60. For example,
71	ould be set to 0 or more than 60. For example, if `APICAST_CON
71	ONFIGURATION_CACHE` is set to 120, the gateway will reload the
71	eload the configuration every 2 minutes (120 seconds).
71	onfiguration every 2 minutes (120 seconds).
75	Default: "127.0.0.1"
77	ning Redis instance for OAuth 2.0 flow. REDIS_HOST parameter
81	Default: 6379
83	ning Redis instance for OAuth 2.0 flow. REDIS_PORT parameter
89	ning Redis instance for OAuth 2.0 flow. REDIS_URL parameter c
89	e used to set the full URI as DSN format like: `redis://PASSWOR
93	pty, the DNS resolver will be autodiscovered.
140	Setting it to particual version will make it not auto

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