Feature/cdap 497 service cli methods

This PR adds two new CLI commands:

``call service <service-id> <method> <path> [headers <%s>] [body <%s>]``
&
``list endpoints service <service-id>`` (Any suggestions for a better command?)

The tab-completion for the Service name works only after https://github.com/caskdata/cdap/pull/560 gets merged. 

Currently, we can not do auto completion for service endpoints till https://issues.cask.co/browse/CDAP-506 gets fixed. 


Should be using `ServiceClient` - at least to get the URL.