Add a command to purge the relay chain only (#306)

* Add a command to purge the relay chain only

* WIP

* Update rococo-parachains/src/cli.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Move cli stuff to its own crate

* Copyright dates

* Test not working for some reason...

* WIP

* Revert "WIP"

This reverts commit f97cd63742c7df822e4a6e52a29db5e0f56b7bfa.

* Fix test to use provided relay chain

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Add hint about which database could not be purged

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

Cargo.lock

@@ -1112,6 +1112,15 @@ dependencies = [
  "rand 0.7.3",
 ]
 
+[[package]]
+name = "cumulus-cli"
+version = "0.1.0"
+dependencies = [
+ "sc-cli",
+ "sc-service",
+ "structopt",
+]
+
 [[package]]
 name = "cumulus-client-collator"
 version = "0.1.0"
@@ -7077,6 +7086,7 @@ name = "rococo-collator"
 version = "0.1.0"
 dependencies = [
  "assert_cmd",
+ "cumulus-cli",
  "cumulus-client-collator",
  "cumulus-client-consensus-relay-chain",
  "cumulus-client-network",
@@ -7133,6 +7143,7 @@ dependencies = [
  "substrate-build-script-utils",
  "substrate-test-client",
  "substrate-test-runtime-client",
+ "tempfile",
  "tokio 0.2.24",
  "trie-root 0.15.2",
 ]