Thin pool support (#269)

• Argument validator extension

Thin provisioning requires yaml input parameters to be checked for
various unsupported combinations.
This commit provides framework that allows searching and denying predefined
parameter combinations.
Checks are performed at the beginning of the role run before blivet
initialization.

• Thin pool support (WIP)

Storage role support for thin provisioning.
Volumes under LVM pool now can use three new options:
- 'thin' - type bool
- setting to True puts the volume into a thin pool
- 'thin_pool_name' - type str
- specifies the name of the thin pool for this volume
- selects thin pool with the same name if it exists
- tries to create new thin pool if it does not
- if omitted and no existing thin pool is present, the name is generated automatically
- if omitted and exactly one thin pool is present, the volume is put into existing thin pool
- if omitted and more than one thin pool is present, exception is raised
- 'thin_pool_size' - type Size
- specifies size of the thin pool

LVM RAID raid0 level support (#272)

• Add workaround for missing LVM raid0 support in blivet

Blivet supports creating LVs with segment type "raid0" but it is
not in the list of supported RAID levels. This will be fixed in
blivet, see storaged-project/blivet#1047

• Add a test for LVM RAID raid0 level

• README: Remove "striped" from the list of supported RAID for pools

We use MD RAID for RAIDs on the pool level which doesn't support
"striped" level.

• README: Clarify supported volume RAID levels

We support different levels for LVM RAID and MD RAID.