…argetpool: multiple bugfixes

Fix apache#1:

targetpool = driver.ex_get_targetpool('tpname')
targetpool.add_node(node)
node.destroy() # targetpool still contains the node
targetpool = driver.ex_get_targetpool('tpname')
    # targetpool.nodes contains node uri string (in addition to possible other node objects)
    # as produced by _to_targetpool because the node is in the pool but is destroyed
targetpool.remove_node(node) # raises an exception, removes the node nevertheless.

Expected behavior: remove the node, return true

Fix apache#2:
targetpool.add(node)
targetpool.add(node)
    # targetpool.nodes contains 2 copies of node
    # actual targetpool resource on the GCE side doesn't contain 2 copies

Expected behavior: no duplicates in targetpool.nodes, the node list matches GCE side

Fix/Improvement apache#3:
Allow specifying nodes by fully-qualified node uri in add_node and remove_node.

if tp.nodes:
  tp.remove_node(tp.nodes[0]) # fails if the node in the list doesn't exist

exoected behavior: should be able to remove any node from the list

GCE allows adding non-existent nodes to the targetpool and doesn't automatically
remove nodes from the pool if you delete them. libcloud should support doing the same.

…hcheck.

The requests were malformed and thus did nothing.

…al addresses to static ones.

…tests; fix documentation and test fixtures for health checks

…s = [], preserve behavior for sourceRanges = None, preserve default value.

GCE documentation states that firewall allows traffic
if it matches either sourceRanges or sourceTags values.
Thus, sourceRanges = [] is a valid parameter value.

Although it would be better to simply set ["0.0.0.0/0"]
as the default, for minimal API breakage, None value is
still supported.