Add support for configuring NFS shares to LAVA

[asac, Feb 24, 2012]: nfs is deferred as its not essential for the current fastmode/big.LITTLE effort. having nfs at some point is a nice to have, put this in backlog with low-medium priority.
[zkrynicki]: this is implemented and done, I'll land it and leave the bp as is
[pwlars 2012-24-24] yes, as we discussed previously, NFS (though originally the recommended approach) is not the way forward on this, and we've proven we can do it without. Go ahead an put the code somewhere in case this is useful again someday, but I've unmilestoned this and dropped it as a requirement for fast models
[zkrynicki, 2012-05-18] moved to lava-core, de-assigned myself, could be relevant for KVM testing again

Meta:
Headline: LAVA has APIs for using NFS
Acceptance:
 1. Users in a particular group can freely create NFS shares without being root
 2. Simple command line tools available (for troubleshooting and administration)
 3. Documented API available for developers

zkrynicki, 2012-02-23: This code will be a part of lava-arm-fast-model until we need to use it elsewhere. This will greatly simplify management at the cost of small "correctness"

Work Items:
Implement basic NFS API: DONE
Implement basic command line tools: DONE
Write documentation for the API: INPROGRESS
Add support for delegating NFS privileges to a dedicated service: TODO
Write upstart script for the NFS service: TODO
Test the service in the private LAVA cloud: TODO