not needed checks are executed on `pod install`

[Fatme]

Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project):

• CLI: 6.1
• Cross-platform modules:
• Android Runtime:
• iOS Runtime:
• Plugin(s):

Describe the bug
Currently CLI executes unneeded checks when executing pod install:

• which pod
• which xcodeproj
• xcproj -project when CocoaPods version is lower than 1.0.0 and Xcode version is 7.3+

To Reproduce
tns prepare ios when there is a plugin with CocoaPods