Closes-Bug: #1390827

spawn storage monitors on selected storage nodes

Issues:
1. As the storage node increases monitor data
communication increases.
2. In libvirt, buffer size of boot command arguments are
limited. As the mon increases, there will be increase
in buffer space and that can cause config overflow issue.

Fix:
1. Added "storage_compute_mon_list" option in testbed.py to
specify monitors that needs to spawned on selected
storage compute nodes.
2. Maximum storage monitors set to 10.
3. If this "storage_compute_mon_list" option is not specified
then storage monitors will be spawned on openstack nodes
and remaining will be spawned on the storage-compute role
configured nodes.

Tests:
1. verified setup_storage with "storage_compute_mon_list"
option enabled/disabled
2. verified add_storage_node with "storage_compute_mon_list"
option enabled/disabled

Change-Id: I19e22ce971b6c4e21c4f1a6a9e7d776abfe9b2ba
Merge "Closes-Bug: #1390827" into R2.0