Changes to perforce client spec (without changing Bamboo) can cause odd behaviour

There are two main cases here that affect us (that I know of):

1. The perforce client has a name change.
If the client name has changed and Bamboo tried to run a command against the now non-existant client, perforce will actually create a client for you. The default for this client is to have the root directory set to the running directory. This can then lead to weird situations where Bamboo is trying to clear out directories that it shouldn't (see https://support.atlassian.com/browse/BSP-1012 for an example)

2. The perforce client has its depot mappings changed
Whilst i can not reproduce this, it is reported that this can actually cause Bamboo to pick up/detect all Perforce changes whether they are related to that build or not.

We need to be a bit more defensive to ensure this does not have any undesirable side effects.