Changes necessary to support netctl login

Changes:

- default http client is replaced by a http.Client in the ContivClient struct
- httpGet()/httpPost()/httpDelete() are moved into ContivClient and use the new internal http.Client
- added SetHttpClient() which allows for overriding the internal http.Client (to disable cert checking, add timeouts, etc.)
- added Login() function which performs a login POST request against auth_proxy
- added SetAuthToken() which adds a custom request header (X-Auth-Token) before requests are sent out
- httpGet()/httpPost()/httpDelete() are modified to set any custom request headers before performing the request
- added `make systemtests` target which runs everything under the systemtests dir

Signed-off-by: Bill Robinson <dseevr@users.noreply.github.com>

Makefile

@@ -1,5 +1,5 @@
 
-.PHONY: all build godep modelgen
+.PHONY: all build godep modelgen systemtests
 
 all: build
 
@@ -11,3 +11,7 @@ godep:
 
 modelgen:
 	@if [ -z "`which modelgen`" ]; then go get -v github.com/contiv/modelgen; fi
+
+# systemtest runs all of the systemtests
+systemtests:
+	go test -v -timeout 5m ./systemtests -check.v