services: add proxy servlet

a proxy servlet that proxy into url appending query string.

example of usage:

<servlet>
  <servlet-name>google1</servlet-name>
  <servlet-class>org.ovirt.engine.core.services.ProxyServlet</servlet-class>
  <init-param>
    <param-name>url</param-name>
    <param-value>https://www.google.com/</param-value>
  </init-param>
  <init-param>
    <param-name>trustStoreType</param-name>
    <param-value>%{ENGINE_PKI_TRUST_STORE_TYPE}</param-value>
  </init-param>
  <init-param>
    <param-name>trustStore</param-name>
    <param-value>%{ENGINE_PKI_TRUST_STORE}</param-value>
  </init-param>
  <init-param>
    <param-name>trustStorePassword</param-name>
    <param-value>%{ENGINE_PKI_TRUST_STORE_PASSWORD}</param-value>
  </init-param>
</servlet>
<servlet-mapping>
  <servlet-name>google1</servlet-name>
  <url-pattern>/google1</url-pattern>
</servlet-mapping>

Change-Id: Ia26768ec7a0e06a20f657cae40f6a51303970b44
Bug-Url: https://bugzilla.redhat.com/1116851
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>