Remove CentOS platform support

CentOS stemcells have been removed upstream, so this cleans up
the remaining CentOS-specific code from the agent:

- Delete platform/net/centos_net_manager.go and its tests
- Remove NewCentOSCertManager from platform/cert/cert_manager.go
- Remove CentOS test context from cert_manager_test.go
- Remove centos platform entry from platform/provider.go
- Clean up CentOS mention in routes_searcher comment

Fixes #339

Author: neddp

platform/cert/cert_manager.go

@@ -59,18 +59,6 @@ func NewUbuntuCertManager(fs boshsys.FileSystem, runner boshsys.CmdRunner, timeo
 	}
 }
 
-func NewCentOSCertManager(fs boshsys.FileSystem, runner boshsys.CmdRunner, timeout time.Duration, logger logger.Logger) Manager {
-	return &certManager{
-		fs:            fs,
-		runner:        runner,
-		path:          "/etc/pki/ca-trust/source/anchors/",
-		updateCmdPath: "/usr/bin/update-ca-trust",
-		logger:        logger,
-		logTag:        "CentOSCertManager",
-		updateTimeout: timeout,
-	}
-}
-
 func NewDummyCertManager(fs boshsys.FileSystem, runner boshsys.CmdRunner, timeout time.Duration, logger logger.Logger) Manager {
 	return &certManager{
 		fs:            fs,

platform/cert/cert_manager_test.go

@@ -339,36 +339,6 @@ var _ = Describe("Certificate Management", func() {
 			})
 		})
 
-		Context("CentOS", func() {
-			BeforeEach(func() {
-				fakeFs = fakesys.NewFakeFileSystem()
-				fakeCmdRunner = fakesys.NewFakeCmdRunner()
-				fakeCmdRunner.AddCmdResult("/usr/bin/update-ca-trust", fakesys.FakeCmdResult{
-					Stdout:     "",
-					Stderr:     "",
-					ExitStatus: 0,
-					Sticky:     true,
-				})
-				certManager = cert.NewCentOSCertManager(fakeFs, fakeCmdRunner, 0, log)
-			})
-
-			SharedLinuxCertManagerExamples("/etc/pki/ca-trust/source/anchors", "/usr/bin/update-ca-trust")
-
-			It("executes update cert command", func() {
-				fakeCmdRunner = fakesys.NewFakeCmdRunner()
-				fakeCmdRunner.AddCmdResult("/usr/bin/update-ca-trust", fakesys.FakeCmdResult{
-					Stdout:     "",
-					Stderr:     "",
-					ExitStatus: 2,
-					Error:      errors.New("command failed"),
-				})
-				certManager = cert.NewCentOSCertManager(fakeFs, fakeCmdRunner, 0, log)
-
-				err := certManager.UpdateCertificates(cert1)
-				Expect(err).To(HaveOccurred())
-			})
-		})
-
 		Context("Windows", func() {
 			const validCerts string = `-----BEGIN CERTIFICATE-----
 MIIC0jCCAboCCQCuQJScK+G0WzANBgkqhkiG9w0BAQsFADArMQswCQYDVQQGEwJV