Add the ability to define a DNS alias

/kind feature

Describe the solution you'd like

I looks like clusterapi sets up an ELB (for example https://pony-unicorns-apiserver-0123456789.us-west-1.elb.amazonaws.com:6443). I would like the ability to define a "nice" DNS name in my route53; for example api.example.com (which then just points as an alias or a CNAME to the ELB)

Anything else you would like to add:

At first, clusterapi can only create the certs for the DNS name (api.example.com); I can then manually point that in DNS (either router53 or otherwise). Ideally though; it would be nice to have cluster-api create it all for me on router53.

Environment:

• Cluster-api-provider-aws version:

Version Info: GitReleaseTag: "v0.2.1", MajorVersion: "0", MinorVersion:"2", GitReleaseCommit:"d0ed4d33a9e433", GitTreeState:"clean"

• Kubernetes version: (use kubectl version):

kubectl version
Client Version: version.Info{Major:"1", Minor:"14", GitVersion:"v1.14.0", GitCommit:"641856db18352033a0d96dbc99153fa3b27298e5", GitTreeState:"clean", BuildDate:"2019-03-25T15:53:57Z", GoVersion:"go1.12.1", Compiler:"gc", Platform:"linux/amd64"}

• OS (e.g. from /etc/os-release):

cat /etc/os-release 
NAME=Fedora
VERSION="30 (Workstation Edition)"
ID=fedora
VERSION_ID=30
VERSION_CODENAME=""
PLATFORM_ID="platform:f30"
PRETTY_NAME="Fedora 30 (Workstation Edition)"
ANSI_COLOR="0;34"
LOGO=fedora-logo-icon
CPE_NAME="cpe:/o:fedoraproject:fedora:30"
HOME_URL="https://fedoraproject.org/"
DOCUMENTATION_URL="https://docs.fedoraproject.org/en-US/fedora/f30/system-administrators-guide/"
SUPPORT_URL="https://fedoraproject.org/wiki/Communicating_and_getting_help"
BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_BUGZILLA_PRODUCT="Fedora"
REDHAT_BUGZILLA_PRODUCT_VERSION=30
REDHAT_SUPPORT_PRODUCT="Fedora"
REDHAT_SUPPORT_PRODUCT_VERSION=30
PRIVACY_POLICY_URL="https://fedoraproject.org/wiki/Legal:PrivacyPolicy"
VARIANT="Workstation Edition"
VARIANT_ID=workstation