Fix Host field in HTTP request when using pdns provider

* add Istio Gateway Source

* add documentation for Istio Gateway Source

* make both istio namespace and ingress gateway service configurable

* prefix gateway types, constructors, and flags with 'istio-'

* fix: add missing sources to source flag docs

The Host field of the HTTP request is set incorrectly when passing
the configured server.  The underlying HTTP library experts a hostname
to be passed.  When passing it a string of the form 'http://pdns.example.com

'
this gets converted into 'http:' (stripping everything from the first
slash onward).  However setting the Host field in the pdnsClientConfig
object is not needed because the underlying library will then parse the
hostname from the URL.

Fixes #661
Signed-off-by: Bartel Sielski <bartel.sielski@gmail.com>

CRD source based on getting endpoints from CRD

Alibaba Cloud Provider add zone info to deployment

Add filter by service type feature

Add Alibaba Cloud Provider

* Add generic metrics for Source & Registry Errors

Change-Id: Ie145f200f815712ec372fca1c897bee3710b045b

* add label to distinguish read errors

Change-Id: I9fb1295c6fb3b2c5814ba942b4af91e1fca1c2c1

* detect write errors

Change-Id: I524360923359a5aebe6224c4e32553b138dfbdab

* revert back to label less metrics

Change-Id: Id689e0ba8c3a40a520f5af73219a4e202c167e03

* fixup Gopkg.lock

Change-Id: I3e3fdc8ad792128421b92db587bdde6e788fea58

Updating changelog for new release v0.5.5

Dyn: cache records per zone using zone's serial number

exoscale: bump version to 0.11

Set Linode user agent

Configure req timeout calling k8s APIs

Signed-off-by: Yoan Blanc <yoan.blanc@exoscale.ch>

When running in a pod sometimes the request to get ingreses/services
stalls indefinitely. A simple pod restart fixes this. Hard to reproduce
but I got lucky and did thread dump which revealed a gorouting blocked
on call to k8s.

What's new is a `--request-timeout` flag that makes requests to k8s
bounded in time. The default is 30s - this may cause some deployments
with a slow api-server to timeout.

adding assume role to aws_sd provider

Linode Provider Implementation

The only thing preventing use of a smaller interval is the API request
limit. Caching records by the zone's serial number would let users set a
smaller interval and still not hit Dyn's request limit if there aren't
any changes to the zone since the last time external-dns has run.

In a dynamic setting bigger interval is still the main throttling
mechanism.