Change return value of GvmClient#getCandidates to String
Refactor GvmClient#getVersionsFor

- renamed to getRemoteVersionsFor
- changed the return type to List<String>
Refactor GvmClientSpec method names
Rename GvmClient#getCandidates to getRemoteCandidates
Add configurable gvmHome to GvmClient
Merge remote-tracking branch 'ollin/master'