Fixes to run Xcode build w/o resigning the app directory contents.

Some changes were required in order to make the plug-in work for our project.

**Dependency on infoplistModify is added to xcodebuild only when
infoplist settings are defined.**

Now dependency to *infoplist-modify* task is added to *xcodebuild* task only if there is infoplist configuration defined in the project. GitHub history shows that it used to work like that but then this logic was removed.

We need it for a reason not directly related to code signing. *INFOPLIST_FILE* property is defined as *$(SRCROOT)/ProjectName/SupportingFiles/ProjectName-Info.plist* in our project. *InfoPlistModifyTask* does not resolve the *SRCROOT* variable and hence fails to read the file.

This problem can be solved by using the following command to read project settings:
```xcodebuild -showBuildSettings|grep INFOPLIST_FILE```

We decided that it is more reasonable to just skip the *InfoPlistModify* task because we don't need any changes in that file.

**ProvisioningCleanup now checks if directory exists before removing it.**

*ProvisioningInstallTask* is skipped when mobileProvisionURI is not defined. ProvisioningCleanup does not find a directory to remove and fails. Now it just reports that directory does not exists and skips the removal.