Provide a means to define a path-prefix for the UI

[tiffon]

With jaegertracing/jaeger-ui#198, a path prefix for the UI can be configured dynamically, e.g. the Jaeger UI can be rooted at /some-prefix/ instead of the root path.

This is to address jaegertracing/jaeger-ui#42.

In order to configure path-prefix, the query-service needs to replace the value of a <base> element in the index.html provided by the UI.

The text to match is:

<base href="/" data-inject-target="BASE_URL"/>

The portion to replace is the "/" value to the href attribute.

For instance, if the path-prefix is "/tracing/east/", the resultant text should be:

<base href="/tracing/east/" data-inject-target="BASE_URL"/>

Note: The data-inject-target="BASE_URL" is only there to serve as some indication that the element is manipulated programmatically, i.e. don't edit manually.

The user needs some way to define the path-prefix. Seems like a CLI param or envvar would be good candidates. The query-service would then use the user-provided value to do the replacement.

A quick note on the format of the value: The first and last characters should be /. If the first is omitted then the base URL will be relative to whatever the current URL is, e.g. http://localhost/trace/abc/tracing/east/ instead of http://localhost/tracing/east/. If the last char is not a / then relative URLs (mostly the static assets referenced in index.html) will not have a separating / from the base URL.