core: Make the number of concurrent vds and irs connections configurable

Allow tuning of apache HttpClients MaxConnectionsPerHost and
MaxTotalConnections values, to increase command throughput.

- Add VdsMaxConnectionsPerHost and IrsMaxConnectionsPerHost config
values. Each value is used for specific type of the connection, with
default 2, which is the value in use without tuning
- Add MaxTotalConnections config value which determines maxiumum number
of allowed connections, with default 20, which is the value in use
without tuning


Change-Id: Id6a7e2bc0f6865e41dd85ab1fd188625cab22ddf
Bug-Url: https://bugzilla.redhat.com/1168538
Signed-off-by: Tim Speetjens <tim.speetjens@redhat.com>
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>