Improve error message when --tag flag is missing

Tag flag (`--tag`) should be treated as required when project actually uses `{{ .Tag }}` placeholder. Otherwise it should be ignored.

#### Steps to reproduce the issue

```sh
go build cmd/deploy/*
cd examples/simple
../../deploy -e local
```

#### What's the expected result?

```
Missing required flag: --tag
```

#### What's the actual result?

```
Value for {{.tag}} placeholder is not specified, available placeholders:
.environment.vars.variable, .service.dir, .service.name, .project.dir, .project.name, .project.vars.variable, .environment.dir, .environment.name
```