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This commit adds ability to use TLS transport for etcd.
New logic is applied when the etcd URL has https:// scheme.
TLS parameters are passed in the environment variables:

ETCD_CA_FILE - path to CA certificate. If not specified, then
system-provided certificates are used.

ETCD_CERT_FILE - client certificate
ETCD_KEY_FILE - client key file
- either both of none of this two must be specified

ETCD_TLS_SERVER_NAME - expected CN of the certificate. Useful when
URL points to a different domain from that in server certificate

ETCD_TLS_INSECURE - if set to "1" (or "true" or "yes") makes client
bypass server certificate validation.

Also for unification with other providers and rest of connection
settings, etcd URL is no longer specified in the command line, but
rather in ETCD_URLS environment variable (defaults to
http://localhost:2379). More than one comma-separated URL can be
specified. All of the URLs must start with either http:// or https://

Also, now it possible to communicate with etcd through proxy specified
in standard environment variables