Use dashes in standalone scripts

We were using underscores and dashes in the script names
inconsistently (somewhere one elsewhere the other).

This moves all the scripts that are mostly meant to be run on their
own (like `get_rhcos_image`) to use dashes (which are easier to type
on most keyboards).

The main numbered scripts are left alone for now.

Author: tomassedovic

Diff for: 03_configure_undercloud.sh

@@ -38,7 +38,7 @@ if ! openstack image show cirros; then
     openstack image create cirros --container-format bare --disk-format qcow2 --public --file "$CIRROS_IMAGE_FILENAME"
 fi
 
-./get_rhcos_image.sh
+./get-rhcos-image.sh
 RHOS_IMAGE_HASH=$(sha512sum $RHCOS_IMAGE_FILENAME | awk '{print $1}')
 if ! openstack image show rhcos; then
     openstack image create rhcos --container-format bare --disk-format qcow2 --public --file $RHCOS_IMAGE_FILENAME

Diff for: README.md

@@ -34,7 +34,7 @@ script will add an `/etc/hosts` entry for the floating IP of the service VM
 hosting the API load balancer.  This is required for the installer to be able
 to look up the API hostname and talk to the API.
 
-- `./expose_ocp_api.sh`
+- `./expose-ocp-api.sh`
 
 ### Customizing Deployment
 
@@ -53,6 +53,6 @@ Once you have a complete working environment, you do not need to re-run all
 sripts.  If you're making changes to the installer, your workflow would look
 like:
 
-- `ocp_cleanup.sh`
+- `ocp-cleanup.sh`
 - `build_ocp_installer.sh`
 - `06_run_ocp.sh`