Add support for handling multiple RMs sans Hadoop config

[kevin-bates]

Alternate address and port parameters can now be passed to the ResourceManager
constructor. This has been facilitated via a keyword args dict (kwargs) to
accommodate future parameters. If both address and alt_address are provided,
address and port will be used to determine if the RM is active. If not, then
alt_address and alt_port will be used. If the latter is active, the address
and port member variables are reset to reflect the alternates.

Also added method get_active_host_port() to return the address, port tuple currently
in use for the ResourceManager instance. This is helpful to clients in cases where
the address was None or HA is configured.

Fixes: #9

EDIT: Removed use of kwargs per review comment.

[lresende]

While looking into this, a couple of things comes to mind:

• In the case of reading the configuration from HADOOP_CONF we need to identify HA related information such as if standby node has been configured and set it properly on the yarn-client-api metadata.
• Once we have the proper configuration and the client establishes the proper master at initialization time, we need to handle the case where the standby had been activated due to master failure and, during API invocation, start using the new master.

Having said that, this might be out of the scope of this initial PR and I would be ok to have this as a new open issue for a later time.

[kevin-bates]

@lresende - this has been rebased with master to include recent merge. (Thanks @dimon222)

[lresende]

LGTM