Use double quotes instead of single quotes for INSTALL_K3S_EXEC

## Expected Behaviour
When installing k3s via k3sup the k3s extra args passed via `--k3s-extra-args` should be interpolated on the remote machine.

## Current Behaviour
Currently the k3s extra args are not interpolated on the remote machine.

## Possible Solution
Change single quotes to escaped double quotes in [install.go](https://github.com/alexellis/k3sup/blob/5a636dba10e1f8e6bb4bb5982c6e04fc21c34534/cmd/install.go#L93)

## Steps to Reproduce (for bugs)
```bash
k3sup install \
--ip $IP \
--user $USER \
--ssh-key $SSH_KEY \
--k3s-version $K3S_VERSION \
--k3s-extra-args "--disable-cloud-controller --no-deploy servicelb --no-deploy traefik --no-deploy local-storage --kubelet-arg='cloud-provider=external' --kubelet-arg=\"provider-id=\"aws:///$(curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone)/$(curl -s http://169.254.169.254/latest/meta-data/instance-id)\"\" --kubelet-arg='feature-gates=CSINodeInfo=true,CSIDriverRegistry=true,CSIBlockVolume=true' --kube-apiserver-arg='allow-privileged=true' --kube-apiserver-arg='feature-gates=CSINodeInfo=true,CSIDriverRegistry=true,CSIBlockVolume=true,VolumeSnapshotDataSource=true' "
```

## Context
When running k3s on an ec2 instance with the aws cloud controller manager, a provider ID has to be set on the k3s instance.

This provider id is determined dynamically on the remote host and consists of the availability zone and instance id.

## Your Environment

* What Kubernetes distribution are you using (for `k3sup app`)?

```
kubectl version
```

```
Client Version: version.Info{Major:"1", Minor:"18", GitVersion:"v1.18.2", GitCommit:"52c56ce7a8272c798dbc29846288d7cd9fbae032", GitTreeState:"clean", BuildDate:"2020-04-16T23:34:25Z", GoVersion:"go1.14.2", Compiler:"gc", Platform:"darwin/amd64"}
```