Allow usage of HTTP_PROXY and HTTPS_PROXY.

[stevesg]

These variables are supported if the default http.Client and http.Transport
are used, but due to custom instances being used, the support is lost. This
commit manually adds it back.

To validate, I tried setting the variables to bogus values to see if the requests would fail:

Before fix, the proxy setting is ignored:

HTTPS_PROXY=http://localhost:9999/ ./cmd/cortextool/cortextool rules print --address=https://www.example.com/ --id=1
INFO[0000] no rule groups currently exist for this user 

After fix, it is not ignored:

HTTPS_PROXY=http://localhost:9999/ ./cmd/cortextool/cortextool rules print --address=https://www.example.com/ --id=1
ERRO[0000] error during request to cortex api            error="Get \"https://www.example.com/api/v1/rules\": proxyconnect tcp: dial tcp 127.0.0.1:9999: connect: connection refused" method=GET url="https://www.example.com/api/v1/rules"
FATA[0000] unable to read rules from cortex, Get "https://www.example.com/api/v1/rules": proxyconnect tcp: dial tcp 127.0.0.1:9999: connect: connection refused