Disables tests with race conditions affecting CI

GH#1546 - The new build system on Travis CI is erring amongst three
tests. Initial investigating hints that they contain race conditions
in the tests (rather than the code).

We've decided to disable the tests whilst we look to resolve since it is
devaluing the CI for the rest of the project.

tests/aws/models/rds/server_tests.rb

@@ -1,4 +1,6 @@
 Shindo.tests("AWS::RDS | server", ['aws', 'rds']) do
+  # Disabled due to https://github.com/fog/fog/1546
+  pending
 
   model_tests(Fog::AWS[:rds].servers, rds_default_server_params) do
     # We'll need this later; create it early to avoid waiting

tests/aws/requests/rds/instance_tests.rb

@@ -1,4 +1,7 @@
 Shindo.tests('AWS::RDS | instance requests', ['aws', 'rds']) do
+  # Disabled due to https://github.com/fog/fog/1546
+  pending
+
   # random_differentiator
   # Useful when rapidly re-running tests, so we don't have to wait
   # serveral minutes for deleted servers to disappear

tests/hp/models/compute/security_group_tests.rb

@@ -1,4 +1,6 @@
 Shindo.tests("Fog::Compute[:hp] | security_group", ['hp']) do
+  # Disabled due to https://github.com/fog/fog/1546
+  pending
 
   model_tests(Fog::Compute[:hp].security_groups, {:name => 'foggroupname', :description => 'foggroupdescription'}, true)