Custom image name and tag overwritten by default values

[iromli]

Describe the bug

Existing image name and tag in settings.json are overwritten by values from gluu_versions.json.

To Reproduce

Steps to reproduce the behavior:

1. Run pygluu-kubernetes generate-settings
2. When asked to modify image name and tag, choose Y
3. Enter LDAP image tag to 4.2.1_dev
4. Wait until command finished
5. Re-run pygluu-kubernetes generate-settings or pygluu-kubernetes install(no prompts appear)
6. Check the value of LDAP_IMAGE_TAG in settings.json

Expected behavior

The value of LDAP_IMAGE_TAG should be 4.2.1_dev as entered when running generate-settings for the first time.

Additional context

Reading from source code, this line is problematic:

self.settings.update(image_names_and_tags)

as it will (eventually) overwrite existing keys in settings.json.

[iromli]

We can add check for non-empty image name and tag before merging values from gluu_versions.json into settings, for example:

self.settings.update({
    k: v for k, v in image_names_and_tags.items()
    if not self.settings.get(k, "")
})